Documentation: update pagemap with shmem exceptions

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/

Mention the current missing information in the pagemap and alternatives
on how to retrieve it, in case someone stumbles upon unexpected
behaviour.

Link: https://lkml.kernel.org/r/20210923064618.157046-1-tiberiu.georgescu@nutanix.com
Link: https://lkml.kernel.org/r/20210923064618.157046-2-tiberiu.georgescu@nutanix.com
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>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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 present, swapped or none can be deduced with
+the help of lseek and/or mincore system calls.
+
+lseek() can differentiate between accessed pages (present or swapped out) and
+holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
+the pages are backed. For anonymous shared pages, the file can be found in
+``/proc/pid/map_files/``.
+
+mincore() can differentiate between pages in memory (present, including swap
+cache) and out of memory (swapped out or none/non-allocated).
+
 Other notes
 ===========