Re: [PATCH v2 1/1] Documentation: update pagemap with shmem exceptions

[David Hildenbrand]

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@xxxxxxxxxxx>
> Reviewed-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> Reviewed-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
> Reviewed-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
> Reviewed-by: Jonathan Davies <jonathan.davies@xxxxxxxxxxx>
> ---
>   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@xxxxxxxxxx>

--
Thanks,

David / dhildenb