From: David Hildenbrand <david@redhat.com>
To: Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>,
akpm@linux-foundation.org, corbet@lwn.net, peterx@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
Subject: Re: [PATCH v2 1/1] Documentation: update pagemap with shmem exceptions
Date: Mon, 20 Sep 2021 19:36:08 +0200 [thread overview]
Message-ID: <a89b861e-e5ff-32cf-c511-5b17ef60c2dd@redhat.com> (raw)
In-Reply-To: <20210920164931.175411-2-tiberiu.georgescu@nutanix.com>
On 20.09.21 18:49, Tiberiu A Georgescu wrote:
> 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>
> ---
> Documentation/admin-guide/mm/pagemap.rst | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index fb578fbbb76c..ea3f88f3c18d 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -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 swapped or none can be deduced with the
> +lseek system call. For a single page, the algorithm is:
> +
> +0. If the pagemap entry of the page has bit 63 (page present) set, the page
> + is present.
> +1. Otherwise, get an fd to the file where the page is backed. For anonymous
> + shared pages, the file can be found in ``/proc/pid/map_files/``.
> +2. Call lseek with LSEEK_DATA flag and seek to the virtual address of the page
> + you wish to inspect. If it overshoots the PAGE_SIZE, the page is NONE.
> +3. Otherwise, the page is in swap.
> +
> Other notes
> ===========
>
>
Thanks!
Reviewed-by: David Hildenbrand <david@redhat.com>
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-09-20 17:38 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-20 16:49 [PATCH v2 0/1] Documenting shmem as an exception case for the pagemap Tiberiu A Georgescu
2021-09-20 16:49 ` [PATCH v2 1/1] Documentation: update pagemap with shmem exceptions Tiberiu A Georgescu
2021-09-20 17:36 ` David Hildenbrand [this message]
2021-09-20 19:07 ` Peter Xu
2021-09-21 8:52 ` Tiberiu Georgescu
2021-09-21 15:04 ` Peter Xu
2021-09-21 16:08 ` Tiberiu Georgescu
2021-09-21 16:30 ` Peter Xu
2021-09-21 17:09 ` Tiberiu Georgescu
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