linux-kernel - [PATCH v3 0/1] Documenting shmem as an exception case for the pagemap

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Date:   Thu, 23 Sep 2021 06:46:17 +0000
From:   Tiberiu A Georgescu <tiberiu.georgescu@...anix.com>
To:     akpm@...ux-foundation.org, corbet@....net, peterx@...hat.com,
        david@...hat.com, linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     ivan.teterevkov@...anix.com, florian.schmidt@...anix.com,
        carl.waldspurger@...anix.com, jonathan.davies@...anix.com,
        chris.riches@...anix.com,
        Tiberiu A Georgescu <tiberiu.georgescu@...anix.com>
Subject: [PATCH v3 0/1] Documenting shmem as an exception case for the pagemap

This patch follows the discussions on previous documentation patch threads
[1][2]. It presents the exception case of shared memory management from the
pagemap's point of view. It briefly describes what is missing, why it is
missing and alternatives to the pagemap for page info retrieval in user
space.

In short, the kernel does not keep track of PTEs for swapped out shared
pages within the processes that references them. Thus, the proc/pid/pagemap
tool cannot print the swap destination of the shared memory pages, instead
setting the pagemap entry to zero for both non-allocated and swapped out
pages. This can create confusion for users who need information on swapped
out pages.

The reasons why maintaining the PTEs of all swapped out shared pages among
all processes while maintaining similar performance is not a trivial task,
or a desirable change, have been discussed extensively [1][3][4][5]. There
are also arguments for why this arguably missing information should
eventually be exposed to the user in either a future pagemap patch, or by
an alternative tool.

[1]: https://marc.info/?m=162878395426774
[2]: https://lore.kernel.org/lkml/20210920164931.175411-1-tiberiu.georgescu@nutanix.com/
[3]: https://lore.kernel.org/lkml/20210730160826.63785-1-tiberiu.georgescu@nutanix.com/
[4]: https://lore.kernel.org/lkml/20210807032521.7591-1-peterx@redhat.com/
[5]: https://lore.kernel.org/lkml/20210715201651.212134-1-peterx@redhat.com/

Tiberiu A Georgescu (1):
  Documentation: update pagemap with shmem exceptions

 Documentation/admin-guide/mm/pagemap.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

-- 
2.33.0.363.g4c719308ce