Re: [PATCH] Documentation: update pagemap with SOFT_DIRTY & UFFD_WP shmem issue

[Jonathan Corbet <corbet@lwn.net>]

Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com> writes:

> Mentioning the current missing functionality of the pagemap, 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 | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index fb578fbbb76c..627f3832b3a2 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -207,3 +207,9 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
>  always 12 at most architectures). Since Linux 3.11 their meaning changes
>  after first clear of soft-dirty bits. Since Linux 4.2 they are used for
>  flags unconditionally.
> +
> +Note that the page table entries for swappable and non-syncable pages are
> +cleared when those pages are zapped or swapped out. This makes information
> +about the page disappear from the pagemap.  The location of the swapped
> +page can still be retrieved from the page cache, but flags like SOFT_DIRTY
> +and UFFD_WP are lost irretrievably.

Thanks for the patch, please accept my apologies for taking to long to
look at it.

The change seems OK to me, but I think that the memory-management
developers should get a chance to at least look at it before I merge
it.  Could I ask you to submit, including the linux-mm list on CC?

Thanks,

jon