mm, documentation: clarify /proc/pid/status VmSwap limitations for shmem

This series is based on Jerome Marchand's [1] so let me quote the first
paragraph from there:

There are several shortcomings with the accounting of shared memory
(sysV shm, shared anonymous mapping, mapping to a tmpfs file).  The
values in /proc/<pid>/status and statm don't allow to distinguish
between shmem memory and a shared mapping to a regular file, even though
their implications on memory usage are quite different: at reclaim, file
mapping can be dropped or written back on disk while shmem needs a place
in swap.  As for shmem pages that are swapped-out or in swap cache, they
aren't accounted at all.

The original motivation for myself is that a customer found (IMHO
rightfully) confusing that e.g.  top output for process swap usage is
unreliable with respect to swapped out shmem pages, which are not
accounted for.

The fundamental difference between private anonymous and shmem pages is
that the latter has PTE's converted to pte_none, and not swapents.  As
such, they are not accounted to the number of swapents visible e.g.  in
/proc/pid/status VmSwap row.  It might be theoretically possible to use
swapents when swapping out shmem (without extra cost, as one has to
change all mappers anyway), and on swap in only convert the swapent for
the faulting process, leaving swapents in other processes until they
also fault (so again no extra cost).  But I don't know how many
assumptions this would break, and it would be too disruptive change for
a relatively small benefit.

Instead, my approach is to document the limitation of VmSwap, and
provide means to determine the swap usage for shmem areas for those who
are interested and willing to pay the price, using /proc/pid/smaps.
Because outside of ipcs, I don't think it's possible to currently to
determine the usage at all.  The previous patchset [1] did introduce new
shmem-specific fields into smaps output, and functions to determine the
values.  I take a simpler approach, noting that smaps output already has
a "Swap: X kB" line, where currently X == 0 always for shmem areas.  I
think we can just consider this a bug and provide the proper value by
consulting the radix tree, as e.g.  mincore_page() does.  In the patch
changelog I explain why this is also not perfect (and cannot be without
swapents), but still arguably much better than showing a 0.

The last two patches are adapted from Jerome's patchset and provide a
VmRSS breakdown to RssAnon, RssFile and RssShm in /proc/pid/status.
Hugh noted that this is a welcome addition, and I agree that it might
help e.g.  debugging process memory usage at albeit non-zero, but still
rather low cost of extra per-mm counter and some page flag checks.

[1] http://lwn.net/Articles/611966/

This patch (of 6):

The documentation for /proc/pid/status does not mention that the value
of VmSwap counts only swapped out anonymous private pages, and not
swapped out pages of the underlying shmem objects (for shmem mappings).
This is not obvious, so document this limitation.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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