memdump

adapted from memdump and linux/tools/vm/page-types.c

build

you need android ndk to compile it.

ndk-build

see makefile :jni/Android.mk, jni/Application.mk and ndk's documents.

use

after compile, executable will be in ./libs/arm64-v8a/ copy the executable to device, run it.

see test scripts.

design

read a process's vma, memory pages and page table from procfs.

/proc

一个应用有多个进程, 但 user (uid) 是相同的. 每启动一个应用都会有一个新的用户. see https://developer.android.com/training/articles/security-tips 每个应用都被赋予唯一的 UID 和非常有限的权限.

see Documentation/filesystems/proc.txt

对我有用的:

fd             Directory, which contains all file descriptors
maps           Memory maps to executables and library files    (2.4)
mem            Memory held by this process
stat           Process status
statm          Process memory status information
status         Process status in human readable form
wchan          Present with CONFIG_KALLSYMS=y: it shows the kernel function
               symbol the task is blocked in - or "0" if not blocked.
pagemap        Page table
stack          Report full stack trace, enable via CONFIG_STACKTRACE
smaps          an extension based on maps, showing the memory consumption of
               each mapping and flags associated with it
numa_maps      an extension based on maps, showing the memory locality and
               binding policy as well as mem usage (in pages) of each mapping.

maps 每一行即一个内存区域. see maps and proc(5). proc.txt, android's kernel

address           perms offset  dev   inode   pathname
08048000-08056000 r-xp 00000000 03:0c 64593   /usr/sbin/gpm

• address: address space, note: the address can be of 8/10/11/12 hex digits. which is 32/40/44/48 bits.
• perms: r = read, w = write, x = execute, s = shared, p = private(copy on write
• offset: offset into the file or whatever.
• device: device(major:minor)
  • /dev/block/zram0 是多少? 这个能看出来吗?
• inode - If the region was mapped from a file, this is the file number., the inode on that device. 0 indicates that no inode is associated with the memory region, like BSS.
• pathname: 如果是文件映射, 则是文件名, 否则可能是 psudo-paths:
  • [stack]: 进程 main stack
  • [stack:]:线程的 stack.
  • [vdso]:virtual dynamically liked shared object. see vdso(7).
  • 其他, 在 android 上, 有很多一般linux 里没有的 pseudo-paths, 下面是测试了几个应用看到的:
    • [anon:bss]
    • [anon:System property context nodes]
    • [anon:arc4random data]
    • [anon:atexit handlers]
    • [anon:bionic TLS guard]
    • [anon:bionic TLS]
    • [anon:libc_malloc]
    • [anon:linker_alloc]
    • [anon:linker_alloc_lob]
    • [anon:linker_alloc_small_objects]
    • [anon:thread signal stack guard]
    • [anon:thread signal stack]
    • [anon:thread stack guard]
    • [vectors]
    • [anon:] = an anonymous mapping that has been named by userspace.
  • if empty, the mapping is anonymous.

page table

The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags using /proc/kpageflags and number of times a page is mapped using /proc/kpagecount. For detailed explanation, see Documentation/vm/pagemap.txt.

不同 内核的 page flag 可能不一样, 最好看一个 fs/proc/page.c:kpageflags_read() 实现.

pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow userspace programs to examine the page tables and related information by reading files in /proc. There are three components to pagemap:

• /proc/pid/pagemap. This file lets a userspace process find out which physical frame each virtual page is mapped to. It contains one 64-bit value for each virtual page, containing the following data (from fs/proc/task_mmu.c, above pagemap_read):

  • Bits 0-54 page frame number (PFN) if present
  • Bits 0-4 swap type if swapped
  • Bits 5-54 swap offset if swapped
  • Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
  • Bits 56-60 zero
  • Bit 61 page is file-page or shared-anon
  • Bit 62 page swapped
  • Bit 63 page present

  If the page is not present but in swap, then the PFN contains an encoding of the swap file number and the page's offset into the swap. Unmapped pages return a null PFN. This allows determining precisely which pages are mapped (or in swap) and comparing mapped pages between processes. Efficient users of this interface will use /proc/pid/maps to determine which areas of memory are actually mapped and llseek to skip over unmapped regions.

• /proc/kpagecount. This file contains a 64-bit count of the number of times each page is mapped, indexed by PFN.

• /proc/kpageflags. This file contains a 64-bit set of flags for each page, indexed by PFN. The flags are (from fs/proc/page.c, above kpageflags_read): 0. LOCKED

  1. ERROR
  2. REFERENCED
  3. UPTODATE
  4. DIRTY
  5. LRU
  6. ACTIVE
  7. SLAB
  8. WRITEBACK
  9. RECLAIM
  10. BUDDY
  11. MMAP
  12. ANON
  13. SWAPCACHE
  14. SWAPBACKED
  15. COMPOUND_HEAD
  16. COMPOUND_TAIL
  17. HUGE
  18. UNEVICTABLE
  19. HWPOISON
  20. NOPAGE
  21. KSM
  22. THP

Short descriptions to the page flags:

0. LOCKED page is being locked for exclusive access, eg. by undergoing read/write IO

1. SLAB page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator When compound page is used, SLUB/SLQB will only set this flag on the head page; SLOB will not flag it at all.

2. BUDDY a free memory block managed by the buddy system allocator The buddy system organizes free memory in blocks of various orders. An order N block has 2^N physically contiguous pages, with the BUDDY flag set for and only for the first page.

3. COMPOUND_HEAD

4. COMPOUND_TAIL A compound page with order N consists of 2^N physically contiguous pages. A compound page with order 2 takes the form of "HTTT", where H donates its head page and T donates its tail page(s). The major consumers of compound pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. memory allocators and various device drivers. However in this interface, only huge/giga pages are made visible to end users.

5. HUGE this is an integral part of a HugeTLB page

6. HWPOISON hardware detected memory corruption on this page: don't touch the data!

7. NOPAGE no page frame exists at the requested address

8. KSM identical memory pages dynamically shared between one or more processes

9. THP contiguous pages which construct transparent hugepages

   [IO related page flags]

10. ERROR IO error occurred

11. UPTODATE page has up-to-date data ie. for file backed page: (in-memory data revision >= on-disk one)

12. DIRTY page has been written to, hence contains new data ie. for file backed page: (in-memory data revision > on-disk one)

13. WRITEBACK page is being synced to disk

    [LRU related page flags]

14. LRU page is in one of the LRU lists

15. ACTIVE page is in the active LRU list

16. UNEVICTABLE page is in the unevictable (non-)LRU list It is somehow pinned and not a candidate for LRU page reclaims, eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments

17. REFERENCED page has been referenced since last LRU list enqueue/requeue

18. RECLAIM page will be reclaimed soon after its pageout IO completed

19. MMAP a memory mapped page

20. ANON a memory mapped page that is not part of a file

21. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry

22. SWAPBACKED page is backed by swap/RAM

The page-types tool in this directory can be used to query the above flags.

Using pagemap to do something useful:

The general procedure for using pagemap to find out about a process' memory usage goes like this:

1. Read /proc/pid/maps to determine which parts of the memory space are mapped to what.
2. Select the maps you are interested in -- all of them, or a particular library, or the stack or the heap, etc.
3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
4. Read a u64 for each page from pagemap.
5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just read, seek to that entry in the file, and read the data you want.

For example, to find the "unique set size" (USS), which is the amount of memory that a process is using that is not shared with any other process, you can go through every map in the process, find the PFNs, look those up in kpagecount, and tally up the number of pages that are only referenced once.

Other notes:

Reading from any of the files will return -EINVAL if you are not starting the read on an 8-byte boundary (e.g., if you sought an odd number of bytes into the file), or if the size of the read is not a multiple of 8 bytes.