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Documentation: update pagemap with shmem exceptions

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Mentioning the current missing information in the pagemap and alternatives
on how to retrieve it, in case someone stumbles upon unexpected behaviour.

Signed-off-by: Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>
Reviewed-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
Reviewed-by: Florian Schmidt <florian.schmidt@nutanix.com>
Reviewed-by: Carl Waldspurger <carl.waldspurger@nutanix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@nutanix.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20210923064618.157046-2-tiberiu.georgescu@nutanix.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Tiberiu A Georgescu 2021-09-23 06:46:18 +00:00 committed by Jonathan Corbet
parent 92a19d8098
commit 5b32e44e8b
1 changed files with 22 additions and 0 deletions
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Documentation/admin-guide/mm/pagemap.rst
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@ -196,6 +196,28 @@ 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.
Exceptions for Shared Memory
============================
Page table entries for shared pages are cleared when the pages are zapped or
swapped out. This makes swapped out pages indistinguishable from never-allocated
ones.
In kernel space, the swap location can still be retrieved from the page cache.
However, values stored only on the normal PTE get lost irretrievably when the
page is swapped out (i.e. SOFT_DIRTY).
In user space, whether the page is present, swapped or none can be deduced with
the help of lseek and/or mincore system calls.
lseek() can differentiate between accessed pages (present or swapped out) and
holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
the pages are backed. For anonymous shared pages, the file can be found in
``/proc/pid/map_files/``.
mincore() can differentiate between pages in memory (present, including swap
cache) and out of memory (swapped out or none/non-allocated).
Other notes
===========