On Wed, Feb 22, 2017 at 09:12:31AM +0100, Michal Hocko wrote:

On Tue 21-02-17 11:43:43, Johannes Weiner wrote:

quoted

Cgroups currently don't report how much shmem they use, which can be
useful data to have, in particular since shmem is included in the
cache/file item while being reclaimed like anonymous memory.

Add a counter to track shmem pages during charging and uncharging.

Yes this is indeed useful. Accounting shmem to the page cache was a
mistake because this is more than confusing. Sad we cannot fix that.

Agreed, this continues to cause confusion with many Linux users :(

I would have just one concern with this patch. You are relying on
PageSwapBacked check but it looks like we are going to implement
MADV_FREE by dropping this flag. I know we do not support MADV_FREE
on shared mappings but if we ever do then the accounting will become
subtly broken. Can/Should we rely on shmem_mapping() check instead?

Yes, right now we do MADV_FREE only on private pages, so this patch is
safe with Shaohua's changes to how we use PG_swapbacked.

Should we support MADV_FREE on shared mappings in the future, using
shmem_mapping() for memcg accounting won't work unfortunately, because
shared pages are truncated from the page cache before uncharging, and
that clears the page->mapping pointer. However, in that case we could
probably unaccount the pages from shmem at the time of MADV_FREE, when
we clear the PG_swapbacked bit.

Other than that the patch looks good to me.

Thanks!