On Wed, Feb 22, 2017 at 09:12:31AM +0100, Michal Hocko wrote: > On Tue 21-02-17 11:43:43, Johannes Weiner wrote: > > 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! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
