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From:   Jonathan Corbet <corbet@lwn.net>
To:     Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>,
        akpm@linux-foundation.org, peterx@redhat.com, david@redhat.com,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     ivan.teterevkov@nutanix.com, florian.schmidt@nutanix.com,
        carl.waldspurger@nutanix.com, jonathan.davies@nutanix.com,
        chris.riches@nutanix.com,
        Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>
Subject: Re: [PATCH v3 0/1] Documenting shmem as an exception case for the
 pagemap
In-Reply-To: <20210923064618.157046-1-tiberiu.georgescu@nutanix.com>
References: <20210923064618.157046-1-tiberiu.georgescu@nutanix.com>
Date:   Mon, 27 Sep 2021 11:28:08 -0600
Message-ID: <87o88ehqs7.fsf@meer.lwn.net>
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Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com> writes:

> 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(+)

Applied, thanks.

jon