On Wed 22-02-17 07:45:01, Johannes Weiner wrote:

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

quoted

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 :(

quoted

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.

You are right!

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.

Or we can just keep the code as is and add a comment to
madvise_free_single_vma to remind that memcg charging would have to be
handled properly if we want to drop vma_is_anonymous check there. It is
really hard to tell whether we ever get a support for MADV_FREE for
shared pages.

quoted

Other than that the patch looks good to me.

Thanks!

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>