On Wed 22-02-17 07:45:01, Johannes Weiner wrote: > 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. 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. > > 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@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
