Linux获取虚拟地址对应的物理地址

adtxl
2022-12-01 / 0 评论 / 406 阅读 / 正在检测是否收录...

要把一个虚拟地址转换成物理地址,其实就是要知道该虚拟地址所在的虚拟页对应的物理页。知道了物理页,再加上页内偏移量即可。以4KB的页大小为例,一个32位长的虚拟地址,其高20位就称为虚拟页号,低12位就是页内偏移。Linux为每一个进程都维护了一个页表,放在内存中。页表的每一项就是一个虚拟页号对应的物理页号。所以如果能够访问到页表,那么就能够把虚拟地址转换成物理地址。然而,只有在内核态才有权限访问页表,用户态是无权访问的。另外,不同的硬件结构下,页表的定位方式是不同的,而且可能很复杂,涉及多个寄存器。

1. 通过pagemap

根据kernel文档,Documentation/admin-guide/mm/pagemap.rst,每一页对应一个64位的字段,也就是8个字节。比如虚拟地址0x00fe0020,其高20位为0xfe0,也就是其虚拟页号为0xfe0。那么该虚拟页的信息处于/proc/self/pagemap这个文件中偏移量为0xfe0*8=32512的地方。从此处读取一个8字节的数据,先检查最高位'page present',如果是1,那么说明该页处于物理内存中,那么该8字节的第0-54位就是物理页号。假设物理页号是0x40,那么实际的物理地址就是(0x40<<12)+0x20=0x00040020。

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 four 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
      :ref:`Documentation/admin-guide/mm/soft-dirty.rst <soft_dirty>`)
    * Bit  56    page exclusively mapped (since 4.2)
    * Bits 57-60 zero
    * Bit  61    page is file-page or shared-anon (since 3.5)
    * Bit  62    page swapped
    * Bit  63    page present

   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.

   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.

The page-types tool in the tools/vm directory can be used to query the
number of times a page is mapped.

 * ``/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
    23. BALLOON
    24. ZERO_PAGE
    25. IDLE

 * ``/proc/kpagecgroup``.  This file contains a 64-bit inode number of the
   memory cgroup each page is charged to, indexed by PFN. Only available when
   CONFIG_MEMCG is set.

Short descriptions to the page flags
====================================

0 - LOCKED
   page is being locked for exclusive access, e.g. by undergoing read/write IO
7 - 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.
10 - 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.
15 - COMPOUND_HEAD
    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
    (:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`),
    the SLUB etc.  memory allocators and various device drivers.
    However in this interface, only huge/giga pages are made visible
    to end users.
16 - COMPOUND_TAIL
    A compound page tail (see description above).
17 - HUGE
    this is an integral part of a HugeTLB page
19 - HWPOISON
    hardware detected memory corruption on this page: don't touch the data!
20 - NOPAGE
    no page frame exists at the requested address
21 - KSM
    identical memory pages dynamically shared between one or more processes
22 - THP
    contiguous pages which construct transparent hugepages
23 - BALLOON
    balloon compaction page
24 - ZERO_PAGE
    zero page for pfn_zero or huge_zero page
25 - IDLE
    page has not been accessed since it was marked idle (see
    :ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`).
    Note that this flag may be stale in case the page was accessed via
    a PTE. To make sure the flag is up-to-date one has to read
    ``/sys/kernel/mm/page_idle/bitmap`` first.

IO related page flags
---------------------

1 - ERROR
   IO error occurred
3 - UPTODATE
   page has up-to-date data
   ie. for file backed page: (in-memory data revision >= on-disk one)
4 - DIRTY
   page has been written to, hence contains new data
   i.e. for file backed page: (in-memory data revision >  on-disk one)
8 - WRITEBACK
   page is being synced to disk

LRU related page flags
----------------------

5 - LRU
   page is in one of the LRU lists
6 - ACTIVE
   page is in the active LRU list
18 - UNEVICTABLE
   page is in the unevictable (non-)LRU list It is somehow pinned and
   not a candidate for LRU page reclaims, e.g. ramfs pages,
   shmctl(SHM_LOCK) and mlock() memory segments
2 - REFERENCED
   page has been referenced since last LRU list enqueue/requeue
9 - RECLAIM
   page will be reclaimed soon after its pageout IO completed
11 - MMAP
   a memory mapped page
12 - ANON
   a memory mapped page that is not part of a file
13 - SWAPCACHE
   page is mapped to swap space, i.e. has an associated swap entry
14 - SWAPBACKED
   page is backed by swap/RAM

The page-types tool in the tools/vm 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.

Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
always 12 at most architectures). Since Linux 3.11 their meaning changes
after first clear of soft-dirty bits. Since Linux 4.2 they are used for
flags unconditionally.

Linux文件目录中的/proc记录着当前进程的信息,称其为虚拟文件系统。在/proc下有一个链接目录名为self,这意味着哪一个进程打开了它,self中存储的信息就是所链接进程的。self中有一个名为pagemap的文件,专门用来记录所链接进程的物理页号信息。这样通过/proc/pid/pagemap文件,允许一个用户态的进程查看到每个虚拟页映射到的物理页

参考

  1. Linux 获取虚拟地址对应的物理地址
  2. Linux 中通过虚拟地址获取物理地址并锁定
  3. Linux下如何在进程中获取虚拟地址对应的物理地址
0

评论

博主关闭了当前页面的评论