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Reverse Mapping (rmap) in Linux Kernel

The document discusses reverse mapping (rmap) in the Linux kernel, comparing legacy approaches (prior to version 2.6.34) with newer methods that improve performance. It details how physical memory is managed through mapping and unmapping processes, including the handling of copy-on-write (COW) during process forking. The implementation intricacies involve structure representations such as anon_vma and vm_area_struct, along with specific functions that handle page reclamation and memory management.

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Reverse Mapping (rmap) 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 • Mapping & reverse mapping • rmap: legacy approach vs new approach (performance improvement) • Implementation Detail
Mapping & Reverse Mapping Process 1 Process N . . Page Table 1 Page Table N . . Physical Memory Page Frame Process 1 Process N . . RMAP Physical Memory Page Frame Page Table 1 Page Table N reclaim clear pte clear pte 1 2 Reverse mapping Mapping rmap – “clear pte”: check ptep_get_and_clear()
rmap: legacy approach vs new approach (performance improvement) 1. Legacy approach: 2.6.33 or earlier kernel 2. New approach: 2.6.34 or later kernel – High-level overview
anon_vma Page #0 vma Page Table Page #1 Page #999 . . anon_vma Page #0 vma: parent vma: child #1 vma: child #N Page Table Page Table Page Table . . Page #1 Page #999 . . Page #0 Page #0 Page #1 Page #1 Page #999 Page #999 Parent process Parent process & child processes: Some pages may be COWed Fork #N children rmap: 2.6.33 or earlier kernel
rmap: 2.6.33 or earlier kernel - Limitation anon_vma Page #0 vma: parent vma: child #1 vma: child #N Page Table Page Table Page Table . . Page #1 Page #999 . . Page #0 Page #0 Page #1 Page #1 Page #999 Page #999 Parent process & child processes Issue statement
2.6.34 or later kernel – High-level overview anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma Process Legend Pointer Doubly linked list RB-tree: RB node
. . . 2.6.34 or later kernel – parent/child processes interconnection anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma anon_vma_chain anon_vma COW Page #1 COW Page #999 . . anon_vma_chain vma Parent process Child process #1 anon_vma_chain anon_vma anon_vma_chain vma Child process #2 anon_vma anon_vma_chain vma Child process #N Page #0 COW Page #1 Page #999 . . Page #0 Page #1 Page #999 . . COW Page #0 anon_vma_chain . . . Legend Pointer Doubly linked list RB-tree: RB node RB-tree: RB node (possible linked node) Shared page COW page
. . . 2.6.34 or later kernel: example 1 anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma anon_vma_chain anon_vma COW Page #1 COW Page #999 . . anon_vma_chain vma Parent process Child process #1 anon_vma_chain anon_vma anon_vma_chain vma Child process #2 anon_vma anon_vma_chain vma Child process #N Page #0 COW Page #1 Page #999 . . Page #0 Page #1 Page #999 . . COW Page #0 anon_vma_chain . . . Legend Pointer Doubly linked list RB-tree: RB node RB-tree: RB node (possible linked node) Shared page COW page reclaim 1 Traverse path (Check pfn of childrens’ pte = reclaiming page’s one) 2 pfn match?
. . . 2.6.34 or later kernel: example 1 – more detail anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma anon_vma_chain anon_vma COW Page #1 COW Page #999 . . anon_vma_chain vma Parent process Child process #1 anon_vma_chain anon_vma anon_vma_chain vma Child process #2 anon_vma anon_vma_chain vma Child process #N Page #0 COW Page #1 Page #999 . . Page #0 Page #1 Page #999 . . COW Page #0 anon_vma_chain . . . Legend Pointer Doubly linked list RB-tree: RB node RB-tree: RB node (possible linked node) Shared page COW page reclaim 1 Traverse path (Check pfn of childrens’ pte = reclaiming page’s one) 4 2 3 pfn match? pfn match? pfn match?
. . . 2.6.34 or later kernel: example 2 anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma anon_vma_chain anon_vma COW Page #1 COW Page #999 . . anon_vma_chain vma Parent process Child process #1 anon_vma_chain anon_vma anon_vma_chain vma Child process #2 anon_vma anon_vma_chain vma Child process #N Page #0 COW Page #1 Page #999 . . Page #0 Page #1 Page #999 . . COW Page #0 anon_vma_chain . . . Legend Pointer Doubly linked list RB-tree: RB node RB-tree: RB node (possible linked node) Shared page COW page reclaim 1 Traverse path (Check pfn of childrens’ pte = reclaiming page’s one)
. . . 2.6.34 or later kernel: example 2 anon_vma Page #0 Page #1 Page #999 . . anon_vma_chain vma anon_vma_chain anon_vma COW Page #1 COW Page #999 . . anon_vma_chain vma Parent process Child process #1 anon_vma_chain anon_vma anon_vma_chain vma Child process #2 anon_vma anon_vma_chain vma Child process #N Page #0 COW Page #1 Page #999 . . Page #0 Page #1 Page #999 . . COW Page #0 anon_vma_chain . . . Legend Pointer Doubly linked list RB-tree: RB node RB-tree: RB node (possible linked node) Shared page COW page reclaim 1 Traverse path (Check pfn of childrens’ pte = reclaiming page’s one) Do not need to traverse all anon_vma_chain(s)
Implementation Detail 1. How/when to construct/link anon_vma, anon_vma_chain and vm_area_struct A. Let’s start from fork() 2. COW - Detail
vm_area_struct vm_mm vm_ops vm_file anon_vma_chain anon_vma anon_vma_chain vma anon_vma same_vma struct rb_node rb anon_vma struct anon_vma *root struct anon_vma *parent struct rb_root_cached rb_root Interval tree implemented via red-black tree RMAP page mapping page cache address_space i_pages (xarray) i_mmap anonymous page Physical Memory page frame unsigned degree = 2 anon_vma, anon_vma_chain & vm_area_struct - Detail
fork() – anon_vma_clone()
fork() – anon_vma_clone()
fork() – anon_vma_fork()
fork() – anon_vma_fork()
fork(): COW → write fault (write-protected fault)
fork(): COW → write fault (write-protected fault) 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) page_add_new_anon_rmap
fork(): COW - page_add_new_anon_rmap()
Child Process: COW Write fault
rb_root & rb – When/who to use? Interval tree traversal (implemented via red-black tree) for reverse mapping
struct list_head anon_vma_chain – When/who to use? Remove VMA: check unlink_anon_vmas()
rmap: page reclaiming – try_to_unamp() rmap_walk rmap_walk_anon rmap_walk_ksm rmap_walk_file try_to_unmap anon_vma_interval_tree_foreach(…, &anon_vma->rb_root, …) invalid_migration_vma try_to_unmap_one page_mapcount_is_zero try_to_unmap_one page_mapcount_is_zero anon_vma_interval_tree_foreach(…, &mapping->i_mmap, …)
Reference • Understanding the Linux Kernel, 3rd Edition • 【原创】（十五）Linux内存管理之RMAP • 奔跑吧 Linux 內核