Re: [PATCH v4 2/4] mm, proc: account for shmem swap in /proc/pid/smaps

On 10/02/2015 03:35 PM, Vlastimil Babka wrote:

Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed
mappings, even if the mapped portion does contain pages that were swapped out.
This is because unlike private anonymous mappings, shmem does not change pte
to swap entry, but pte_none when swapping the page out. In the smaps page
walk, such page thus looks like it was never faulted in.

This patch changes smaps_pte_entry() to determine the swap status for such
pte_none entries for shmem mappings, similarly to how mincore_page() does it.
Swapped out pages are thus accounted for.

The accounting is arguably still not as precise as for private anonymous
mappings, since now we will count also pages that the process in question never
accessed, but only another process populated them and then let them become
swapped out. I believe it is still less confusing and subtle than not showing
any swap usage by shmem mappings at all. Also, swapped out pages only becomee a
performance issue for future accesses, and we cannot predict those for neither
kind of mapping.

Agreed, this is much better than the current situation. I don't think
there is such a thing as a perfect accounting of shared pages anyway.

Signed-off-by: Vlastimil Babka <redacted>
Acked-by: Konstantin Khlebnikov <redacted>

Acked-by: Jerome Marchand <redacted>