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Commit 83b4b0bb authored by Konstantin Khlebnikov's avatar Konstantin Khlebnikov Committed by Linus Torvalds
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pagemap: update documentation 



Notes about recent changes.

[akpm@linux-foundation.org: various tweaks]
Signed-off-by: default avatarKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Mark Williamson <mwilliamson@undo-software.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 77bb499b

Documentation/vm/pagemap.txt

+12 −2
Original line number Diff line number Diff line
@@ -16,12 +16,17 @@ There are three components to pagemap: 
    * 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)

    * Bit  56    page exclusively mapped

    * Bit  56    page exclusively mapped (since 4.2)

    * Bits 57-60 zero

    * Bit  61    page is file-page or shared-anon

    * 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
@@ -160,3 +165,8 @@ 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.