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pagemap: update documentation 

Notes about recent changes.

[akpm@linux-foundation.org: various tweaks]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Mark Williamson <mwilliamson@undo-software.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Konstantin Khlebnikov 2015-09-08 15:00:13 -07:00 committed by Linus Torvalds
parent 77bb499bb6
commit 83b4b0bb63
1 changed files with 12 additions and 2 deletions
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14
Documentation/vm/pagemap.txt
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@ -16,12 +16,17 @@ There are three components to pagemap:
* Bits 0-4 swap type if swapped * Bits 0-4 swap type if swapped
* Bits 5-54 swap offset if swapped * Bits 5-54 swap offset if swapped
* Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt) * 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 * 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 62 page swapped
* Bit 63 page present * 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 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 encoding of the swap file number and the page's offset into the
swap. Unmapped pages return a null PFN. This allows determining 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 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 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. 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.