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

[Jerome Marchand]

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 <vbabka@xxxxxxx>
> Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

Acked-by: Jerome Marchand <jmarchan@xxxxxxxxxx>

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