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Memory Mapping Implementation (mmap) in Linux Kernel

Adrian Huang
Adrian Huang

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Memory Mapping Implementation (mmap) in Linux Kernel Adrian Huang | May, 2022 * Based on kernel 5.11 (x86_64) – QEMU * SMP (4 CPUs) and 8GB memory * Kernel parameter: nokaslr norandmaps * Userspace: ASLR is disabled * Legacy BIOS
Agenda • Three different IO types • Process Address Space – mm_struct & VMA • Four types of memory mappings • mmap system call implementation • Demand page: page fault handling for four types of memory mappings • fork() • COW mapping configuration: set ‘write protect’ when calling fork() • COW fault call path
Three different IO types • Buffered IO o Leverage page cache • Direct IO o Bypass page cache • Memory-mapped IO (file-based mapping, memory-mapped file) • File content can be accessed by operations on the bytes in the corresponding memory region.
Process Address Space – mm_struct & VMA (1/2)
Process Address Space – mm_struct & VMA (2/2)
Four types of memory mappings Reference from: Chapter 49, The Linux Programming Interface File Anonymous Private (Modification is not visible to other processes) Initializing memory from contents of file Example: Process's .text and .data segements (Changes are not carried through to the underlying file) Memory Allocation Shared (Modification is visible to other processes) 1. Memory-mapped IO: Changes are carried through to the underlying file 2. Sharing memory between processes (IPC) Sharing memory between processes (IPC) Visibility of Modification Mapping Type
mmap system call implementation (1/2)
mmap system call implementation (2/2) 1. thp_get_unmapped_area() eventually calls arch_get_unmapped_area_topdown() 2. unmapped_area_topdown() is the key point for allocating an userspace address
inode_operations & file_operations: file thp_get_unmapped_area(): DAX supported for huge page
inode_operations & file_operations: directory
vma vm_start = 0x400000 vm_end = 0x401000 vma vm_start = 0x401000 vm_end = 0x496000 vma vm_start = 0x496000 vm_end = 0x4bd000 vma vm_start = 0x4be000 vm_end = 0x4c1000 vma vm_start = 0x4c1000 vm_end = 0x4c4000 vma (heap) vm_start = 0x4c4000 vm_end = 0x4e8000 vma (vvar) vm_start = 0x7ffff7ffa000 vm_end = 0x7ffff7ffe000 vma (vdso) vm_start = 0x7ffff7ffe000 vm_end = 0x7ffff7fff000 vma (stack) vm_start = 0x7ffffffde000 vm_end = 0x7ffffffff000 GAP GAP G A P GAP user space address 0 0x7ffffffff000 Process address space: Doubly linked-list
Process address space: VMA rbtree
Process address space: VMA rbtree -> rb_subtree_gap • Maximum of the following items: o gap between this vma and previous one o rb_left.subtree_gap o rb_right.subtree_gap rb_subtree_gap calculation
Process address space: VMA rbtree -> rb_subtree_gap • Maximum of the following items: o gap between this vma and previous one o rb_left.subtree_gap o rb_right.subtree_gap rb_subtree_gap calculation max(0x7ffff7ffa000 – 0x4e8000, 0, 0) = 0x7fff7b12000
Process address space: VMA rbtree -> rb_subtree_gap • VM_GROWSDOWN is set in vma->vm_flags • max(0x7ffffffde000 – 0x7ffff7fff000 - stack_guard_gap, 0, 0) = 0x7edf000 • stack_guard_gap = 256UL<<PAGE_SHIFT
unmapped_area_topdown()
unmapped_area_topdown() - Principle vm_unmapped_area_info length = 0x1000 low_limit = PAGE_SIZE high_limit = mm->mmap_base = 0x7ffff7fff000 gap_end = info->high_limit (0x7ffff7fff000) high_limit = gap_end – info->length = 0x7ffff7ffe000 gap_start = mm->highest_vm_end = 0x7ffffffff000 unmapped_area_topdown(): Preparation [Efficiency] Traverse vma rbtree to find gap space instead of traversing vma linked list
unmapped_area_topdown()
unmapped_area_topdown()
unmapped_area_topdown()
unmapped_area_topdown() Stack guard gap
unmapped_area_topdown() Stack guard gap gap_end = 0x7ffffffde000 – 0x100000 = 0x7fffffede000 * stack_guard_gap = 256UL<<PAGE_SHIFT = 0x100000
unmapped_area_topdown() Not found yet
unmapped_area_topdown() 1 2
unmapped_area_topdown()
unmapped_area_topdown() Found return 0x7ffff7ffa000 – 0x1000 = 0x7ffff7ff9000
vma_merge() – Possible ways to merge Note: VMAs must have the same attributes
mmap_region()->munmap_vma_range()->find_vma_links find_vma_links(…, 0x7ffff7ff9000, …) Iterate rbtree to find an empty VMA link (rb_link) • rb_parent: The parent of rb_link • rb_prev: Previous vma of rb_parent • rb_prev is used in vma_merge() • rb_link, rb_parent and rb_prev are used in vma_link(). • rb_prev is the previous VMA after the new VMA is inserted. Note
vm_area_alloc() & vma_link()
__vma_link_file(): i_mmap for recording all memory mappings (vma) of the memory-mapped file task_struct mm files_struct fd_array[] file f_inode f_pos f_mapping . . file inode *i_mapping i_atime i_mtime i_ctime mnt dentry f_path address_space i_data host page_tree i_mmap page mapping index radix_tree_root height = 2 rnode radix_tree_node count = 2 63 0 1 … page 2 3 radix_tree_node count = 1 63 0 1 … 2 3 page page slots[0] slots[3] slots[1] slots[3] slots[2] index = 1 index = 3 index = 194 radix_tree_node count = 1 63 0 1 … 2 3 Interval tree implemented via red-black tree Radix Tree (v4.19 or earlier) Xarray (v4.20 or later) files mm struct vm_area_struct *mmap get_unmapped_area mmap_base page cache vm_area_struct vm_mm vm_ops vm_file vm_area_struct vm_file . . pgd 1. i_mmap: Reverse mapping (RMAP) for the memory-mapped file (page cache) → check __vma_link_file () 2. anon_vma: Reverse mapping for anonymous pages -> anon_vma_prepare() invoked during page fault RMAP
mm_populate()
Anonymous Page: Page Fault – Discussion List • Private (MAP_PRIATE) • Write • Read before a write • Share (MAP_SHARED)
Demand Paging: page fault – Anonymous page (MAP_PRIVATE)
Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE) Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset
Demand Paging: page fault → Page table configuration [anonymous page: MAP_PRIVATE] page fault flow
man mmap [gdb] backtrace Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE) [anonymous page: MAP_PRIVATE] page fault flow
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE)
Demand Paging: page fault → call trace - Anonymous page (MAP_PRIVATE)
Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) Pre-allocated zero page Physical Memory read fault 1. A pre-allocated zero page is initialized during system init -> Check init_zero_pfn() 2. Anonymous private page + read fault -> Link to the pre-allocated zero page
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) Pre-allocated zero page Physical Memory read fault 1. A pre-allocated zero page is initialized during system init -> Check init_zero_pfn() 2. Anonymous private page + read fault -> Link to the pre-allocated zero page Link to the pre-allocated zero page
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 Dedicated zero page 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) -> Write fault mmap (anonymous) 1 Allocate a zeroed page 2 Link to the newly allocated page write fault
Page fault handler for userspace address
Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE & MAP_SHARED): first-time access Anonymous page + MAP_PRIVATE Anonymous page + MAP_SHARED + WRITE Anonymous page + MAP_SHARED + READ [do_anonymous_page] • Read fault: Apply the pre-allocated zero page
Demand Paging: page fault → VMAs – Anonymous page (MAP_SHARED): first-time write
Demand Paging: page fault → VMAs – Anonymous page (MAP_SHARED): first-time write Anonymous page + MAP_SHARED
Types of memory mappings: update Reference from: Chapter 49, The Linux Programming Interface Description Backing file Private (Modification is not visible to other processes) Initializing memory from contents of file Example: Process's .text and .data segements (Changes are not carried through to the underlying file) Memory Allocation No Shared (Modification is visible to other processes) 1. Memory-mapped IO: Changes are carried through to the underlying file 2. Sharing memory between processes (IPC) Sharing memory between processes (IPC) /dev/zero Mapping Type Anonymous File Visibility of Modification
Memory-mapped file vma->vm_ops: invoke vm_ops callbacks in page fault handler • struct vm_operations_struct o fault : Invoked by page fault handler to read the corresponding data into a physical page. o map_pages : Map the page if it is in page cache (check function ‘do_fault_around’: warm/cold page cache). o page_mkwrite: Notification that a previously read-only page is about to become writable.
Memory-mapped file: related data structures NULL: anonymous page
Page fault handler for userspace address: Memory-mapped file Look at this firstly
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 Demand Paging: page fault → do_read_fault(): warm page cache read page fault Memory-mapped file
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 Demand Paging: page fault → do_read_fault(): warm page cache read page fault Memory-mapped file
Demand Paging: page fault → filemap_map_pages(): warm page cache read page fault 1 2 map_pages callback: map a page cache (already available) to process address space (process page table) → warm page cache Memory-mapped file
Demand Paging: page fault → call path for warm/cold page cache do_read_fault alloc_set_pte filemap_map_pages vmf->vma->vm_ops->map_pages do_fault_around Find the page cache from address_space (page cache pool) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte warm page cache cold page cache find_get_page do_async_mmap_readahead No page in page cache page in page cache
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 warm page cache: make sure “page cache = mmap physical page” (1/3) Physical Memory Page Cache - Verification page->mapping • [bit 0] = 1: anonymous page → mapping field = anon_vma descriptor • [bit 0] = 0: page cache → mapping field = address_space descriptor
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 After alloc_set_pte() After alloc_set_pte() Physical Memory << PAGE_SHIFT warm page cache: make sure “page cache = mmap physical page” (2/3)
Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 After alloc_set_pte() After alloc_set_pte() Physical Memory will generate a write fault for next write warm page cache: make sure “page cache = mmap physical page” (3/3)
write fault (write-protected fault) write fault Memory-mapped file
do_wp_page wp_page_copy new_page = alloc_page_vma(…) cow_user_page copy_user_highpage maybe_mkwrite wp_page_shared [MAP_PRIVATE] COW: Copy On Write [MAP_SHARED] vma is (VM_WRITE|VM_SHARED) write fault (write-protected fault): do_wp_page write fault [MAP_PRIVATE] COW: Quote from `man mmap` Memory-mapped file
write fault (write-protected fault) – call path
write fault without previously reading (MAP_SHARED): do_shared_fault write fault Memory-mapped file
write fault without previously reading (MAP_SHARED): do_shared_fault do_shared_fault __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache do_page_mkwrite fault_dirty_shared_page
write fault without previously reading (MAP_SHARED): do_shared_fault do_shared_fault __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW do_page_mkwrite fault_dirty_shared_page
write fault without previously reading (MAP_PRIVATE): do_cow_fault write fault Memory-mapped file
write fault without previously reading (MAP_PRIVATE): do_cow_fault do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page Quote from `man mmap`
do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault
do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault 2
do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault
[Recap] Think about this: #1 write fault read fault 1 1 2 2 Memory-mapped file
Think about this: #2 fork(): Which function (do_cow_fault or do_wp_page) is called when COW is triggered? Think about this… this one? or, this one?
fork(): COW mapping – set ‘write protect’ for src/dst PTEs
fork(): COW mapping – set ‘write protect’ for src/dst PTEs
fork(): COW Fault Call Path
Backup
vdso & vvar • vsyscall (Virtual System Call) o The context switch overhead (user <-> kernel) of some system calls (gettimeofday, time, getcpu) is greater than execution time of those functions: Built on top of the fixed-mapped address o Machine code format o Core dump: debugger cannot provide the debugging info because symbols of this area are unavailable • vDSO (Virtual Dynamic Shared Object) • ELF format • A small shared library that the kernel automatically maps into the address space of all userspace applications • VVAR (vDSO Variable)
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Memory Mapping Implementation (mmap) in Linux Kernel

  • 1. Memory Mapping Implementation (mmap) in Linux Kernel Adrian Huang | May, 2022 * Based on kernel 5.11 (x86_64) – QEMU * SMP (4 CPUs) and 8GB memory * Kernel parameter: nokaslr norandmaps * Userspace: ASLR is disabled * Legacy BIOS
  • 2. Agenda • Three different IO types • Process Address Space – mm_struct & VMA • Four types of memory mappings • mmap system call implementation • Demand page: page fault handling for four types of memory mappings • fork() • COW mapping configuration: set ‘write protect’ when calling fork() • COW fault call path
  • 3. Three different IO types • Buffered IO o Leverage page cache • Direct IO o Bypass page cache • Memory-mapped IO (file-based mapping, memory-mapped file) • File content can be accessed by operations on the bytes in the corresponding memory region.
  • 4. Process Address Space – mm_struct & VMA (1/2)
  • 5. Process Address Space – mm_struct & VMA (2/2)
  • 6. Four types of memory mappings Reference from: Chapter 49, The Linux Programming Interface File Anonymous Private (Modification is not visible to other processes) Initializing memory from contents of file Example: Process's .text and .data segements (Changes are not carried through to the underlying file) Memory Allocation Shared (Modification is visible to other processes) 1. Memory-mapped IO: Changes are carried through to the underlying file 2. Sharing memory between processes (IPC) Sharing memory between processes (IPC) Visibility of Modification Mapping Type
  • 7. mmap system call implementation (1/2)
  • 8. mmap system call implementation (2/2) 1. thp_get_unmapped_area() eventually calls arch_get_unmapped_area_topdown() 2. unmapped_area_topdown() is the key point for allocating an userspace address
  • 9. inode_operations & file_operations: file thp_get_unmapped_area(): DAX supported for huge page
  • 11. vma vm_start = 0x400000 vm_end = 0x401000 vma vm_start = 0x401000 vm_end = 0x496000 vma vm_start = 0x496000 vm_end = 0x4bd000 vma vm_start = 0x4be000 vm_end = 0x4c1000 vma vm_start = 0x4c1000 vm_end = 0x4c4000 vma (heap) vm_start = 0x4c4000 vm_end = 0x4e8000 vma (vvar) vm_start = 0x7ffff7ffa000 vm_end = 0x7ffff7ffe000 vma (vdso) vm_start = 0x7ffff7ffe000 vm_end = 0x7ffff7fff000 vma (stack) vm_start = 0x7ffffffde000 vm_end = 0x7ffffffff000 GAP GAP G A P GAP user space address 0 0x7ffffffff000 Process address space: Doubly linked-list
  • 13. Process address space: VMA rbtree -> rb_subtree_gap • Maximum of the following items: o gap between this vma and previous one o rb_left.subtree_gap o rb_right.subtree_gap rb_subtree_gap calculation
  • 14. Process address space: VMA rbtree -> rb_subtree_gap • Maximum of the following items: o gap between this vma and previous one o rb_left.subtree_gap o rb_right.subtree_gap rb_subtree_gap calculation max(0x7ffff7ffa000 – 0x4e8000, 0, 0) = 0x7fff7b12000
  • 15. Process address space: VMA rbtree -> rb_subtree_gap • VM_GROWSDOWN is set in vma->vm_flags • max(0x7ffffffde000 – 0x7ffff7fff000 - stack_guard_gap, 0, 0) = 0x7edf000 • stack_guard_gap = 256UL<<PAGE_SHIFT
  • 17. unmapped_area_topdown() - Principle vm_unmapped_area_info length = 0x1000 low_limit = PAGE_SIZE high_limit = mm->mmap_base = 0x7ffff7fff000 gap_end = info->high_limit (0x7ffff7fff000) high_limit = gap_end – info->length = 0x7ffff7ffe000 gap_start = mm->highest_vm_end = 0x7ffffffff000 unmapped_area_topdown(): Preparation [Efficiency] Traverse vma rbtree to find gap space instead of traversing vma linked list
  • 22. unmapped_area_topdown() Stack guard gap gap_end = 0x7ffffffde000 – 0x100000 = 0x7fffffede000 * stack_guard_gap = 256UL<<PAGE_SHIFT = 0x100000
  • 27. vma_merge() – Possible ways to merge Note: VMAs must have the same attributes
  • 28. mmap_region()->munmap_vma_range()->find_vma_links find_vma_links(…, 0x7ffff7ff9000, …) Iterate rbtree to find an empty VMA link (rb_link) • rb_parent: The parent of rb_link • rb_prev: Previous vma of rb_parent • rb_prev is used in vma_merge() • rb_link, rb_parent and rb_prev are used in vma_link(). • rb_prev is the previous VMA after the new VMA is inserted. Note
  • 30. __vma_link_file(): i_mmap for recording all memory mappings (vma) of the memory-mapped file task_struct mm files_struct fd_array[] file f_inode f_pos f_mapping . . file inode *i_mapping i_atime i_mtime i_ctime mnt dentry f_path address_space i_data host page_tree i_mmap page mapping index radix_tree_root height = 2 rnode radix_tree_node count = 2 63 0 1 … page 2 3 radix_tree_node count = 1 63 0 1 … 2 3 page page slots[0] slots[3] slots[1] slots[3] slots[2] index = 1 index = 3 index = 194 radix_tree_node count = 1 63 0 1 … 2 3 Interval tree implemented via red-black tree Radix Tree (v4.19 or earlier) Xarray (v4.20 or later) files mm struct vm_area_struct *mmap get_unmapped_area mmap_base page cache vm_area_struct vm_mm vm_ops vm_file vm_area_struct vm_file . . pgd 1. i_mmap: Reverse mapping (RMAP) for the memory-mapped file (page cache) → check __vma_link_file () 2. anon_vma: Reverse mapping for anonymous pages -> anon_vma_prepare() invoked during page fault RMAP
  • 32. Anonymous Page: Page Fault – Discussion List • Private (MAP_PRIATE) • Write • Read before a write • Share (MAP_SHARED)
  • 33. Demand Paging: page fault – Anonymous page (MAP_PRIVATE)
  • 34. Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE) Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset
  • 35. Demand Paging: page fault → Page table configuration [anonymous page: MAP_PRIVATE] page fault flow
  • 36. man mmap [gdb] backtrace Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE) [anonymous page: MAP_PRIVATE] page fault flow
  • 37. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → Page table configuration - Anonymous page (MAP_PRIVATE)
  • 38. Demand Paging: page fault → call trace - Anonymous page (MAP_PRIVATE)
  • 39. Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 mmap 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset
  • 40. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) Pre-allocated zero page Physical Memory read fault 1. A pre-allocated zero page is initialized during system init -> Check init_zero_pfn() 2. Anonymous private page + read fault -> Link to the pre-allocated zero page
  • 41. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) Pre-allocated zero page Physical Memory read fault 1. A pre-allocated zero page is initialized during system init -> Check init_zero_pfn() 2. Anonymous private page + read fault -> Link to the pre-allocated zero page Link to the pre-allocated zero page
  • 42. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PDE #511 PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PDE #447 PTE #505 Dedicated zero page 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE) – Read before writing data (read fault) -> Write fault mmap (anonymous) 1 Allocate a zeroed page 2 Link to the newly allocated page write fault
  • 43. Page fault handler for userspace address
  • 44. Demand Paging: page fault → VMAs – Anonymous page (MAP_PRIVATE & MAP_SHARED): first-time access Anonymous page + MAP_PRIVATE Anonymous page + MAP_SHARED + WRITE Anonymous page + MAP_SHARED + READ [do_anonymous_page] • Read fault: Apply the pre-allocated zero page
  • 45. Demand Paging: page fault → VMAs – Anonymous page (MAP_SHARED): first-time write
  • 46. Demand Paging: page fault → VMAs – Anonymous page (MAP_SHARED): first-time write Anonymous page + MAP_SHARED
  • 47. Types of memory mappings: update Reference from: Chapter 49, The Linux Programming Interface Description Backing file Private (Modification is not visible to other processes) Initializing memory from contents of file Example: Process's .text and .data segements (Changes are not carried through to the underlying file) Memory Allocation No Shared (Modification is visible to other processes) 1. Memory-mapped IO: Changes are carried through to the underlying file 2. Sharing memory between processes (IPC) Sharing memory between processes (IPC) /dev/zero Mapping Type Anonymous File Visibility of Modification
  • 48. Memory-mapped file vma->vm_ops: invoke vm_ops callbacks in page fault handler • struct vm_operations_struct o fault : Invoked by page fault handler to read the corresponding data into a physical page. o map_pages : Map the page if it is in page cache (check function ‘do_fault_around’: warm/cold page cache). o page_mkwrite: Notification that a previously read-only page is about to become writable.
  • 49. Memory-mapped file: related data structures NULL: anonymous page
  • 50. Page fault handler for userspace address: Memory-mapped file Look at this firstly
  • 51. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 Demand Paging: page fault → do_read_fault(): warm page cache read page fault Memory-mapped file
  • 52. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 Physical Memory PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 Legend Allocated pages or page table entry Will be allocated if page fault occurs PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 Demand Paging: page fault → do_read_fault(): warm page cache read page fault Memory-mapped file
  • 53. Demand Paging: page fault → filemap_map_pages(): warm page cache read page fault 1 2 map_pages callback: map a page cache (already available) to process address space (process page table) → warm page cache Memory-mapped file
  • 54. Demand Paging: page fault → call path for warm/cold page cache do_read_fault alloc_set_pte filemap_map_pages vmf->vma->vm_ops->map_pages do_fault_around Find the page cache from address_space (page cache pool) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte warm page cache cold page cache find_get_page do_async_mmap_readahead No page in page cache page in page cache
  • 55. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 warm page cache: make sure “page cache = mmap physical page” (1/3) Physical Memory Page Cache - Verification page->mapping • [bit 0] = 1: anonymous page → mapping field = anon_vma descriptor • [bit 0] = 0: page cache → mapping field = address_space descriptor
  • 56. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 After alloc_set_pte() After alloc_set_pte() Physical Memory << PAGE_SHIFT warm page cache: make sure “page cache = mmap physical page” (2/3)
  • 57. Page Map Level-4 Table Sign-extend Page Map Level-4 Offset 30 21 39 38 29 47 48 63 Page Directory Pointer Offset Page Directory Offset Page Directory Pointer Table Page Directory Table PML4E #255 PML4E for kernel PDPTE #511 PTE #510 stack task_struct pgd mm mm_struct mmap PTE #509 … PTE #478 PML4E #0 PDE #2 PDPTE #0 PTE #0 .text, .data, … Linear Address: 0x7ffff7ff9000 PTE #188 … PTE #190 PTE #199 … heap PTE #505 mmap (memory-mapped file = page cache) 12 20 11 0 Page Table Page Directory Pointer Offset Page Directory Offset Disk PDE #511 PDE #447 Already in page cache: file->f_mapping 1 2 After alloc_set_pte() After alloc_set_pte() Physical Memory will generate a write fault for next write warm page cache: make sure “page cache = mmap physical page” (3/3)
  • 58. write fault (write-protected fault) write fault Memory-mapped file
  • 59. do_wp_page wp_page_copy new_page = alloc_page_vma(…) cow_user_page copy_user_highpage maybe_mkwrite wp_page_shared [MAP_PRIVATE] COW: Copy On Write [MAP_SHARED] vma is (VM_WRITE|VM_SHARED) write fault (write-protected fault): do_wp_page write fault [MAP_PRIVATE] COW: Quote from `man mmap` Memory-mapped file
  • 60. write fault (write-protected fault) – call path
  • 61. write fault without previously reading (MAP_SHARED): do_shared_fault write fault Memory-mapped file
  • 62. write fault without previously reading (MAP_SHARED): do_shared_fault do_shared_fault __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache do_page_mkwrite fault_dirty_shared_page
  • 63. write fault without previously reading (MAP_SHARED): do_shared_fault do_shared_fault __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW do_page_mkwrite fault_dirty_shared_page
  • 64. write fault without previously reading (MAP_PRIVATE): do_cow_fault write fault Memory-mapped file
  • 65. write fault without previously reading (MAP_PRIVATE): do_cow_fault do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page Quote from `man mmap`
  • 66. do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault
  • 67. do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault 2
  • 68. do_cow_fault vmf->cow_page = alloc_page_vma(…) __do_fault vmf->vma->vm_ops->fault ext4_filemap_fault filemap_fault do_sync_mmap_readahead finish_fault alloc_set_pte find_get_page do_async_mmap_readahead No page in page cache page in page cache: COW copy_user_highpage(vmf->cow_page, vmf->page, …) 1. Apply vmf->page if not a COW page 2. Apply vmf->cow_page if a COW page 1 2 3 4 breakpoint write fault without previously reading (MAP_PRIVATE): do_cow_fault
  • 69. [Recap] Think about this: #1 write fault read fault 1 1 2 2 Memory-mapped file
  • 70. Think about this: #2 fork(): Which function (do_cow_fault or do_wp_page) is called when COW is triggered? Think about this… this one? or, this one?
  • 71. fork(): COW mapping – set ‘write protect’ for src/dst PTEs
  • 72. fork(): COW mapping – set ‘write protect’ for src/dst PTEs
  • 73. fork(): COW Fault Call Path
  • 75. vdso & vvar • vsyscall (Virtual System Call) o The context switch overhead (user <-> kernel) of some system calls (gettimeofday, time, getcpu) is greater than execution time of those functions: Built on top of the fixed-mapped address o Machine code format o Core dump: debugger cannot provide the debugging info because symbols of this area are unavailable • vDSO (Virtual Dynamic Shared Object) • ELF format • A small shared library that the kernel automatically maps into the address space of all userspace applications • VVAR (vDSO Variable)
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