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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pcdxP-00EQIS-Vc; Thu, 16 Mar 2023 03:07:12 +0000 Date: Thu, 16 Mar 2023 03:07:11 +0000 From: Matthew Wilcox To: Yosry Ahmed Cc: Waiman Long , Andrew Morton , Hugh Dickins , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Naoya Horiguchi , Miaohe Lin , Vladimir Davydov , linux-mm@kvack.org, cgroups@vger.kernel.org Subject: Re: [PATCH] memcg: page_cgroup_ino() get memcg from compound_head(page) Message-ID: References: <20230313083452.1319968-1-yosryahmed@google.com> <20230313124431.fe901d79bc8c7dc96582539c@linux-foundation.org> <8d4e1b74-6ae8-4243-d5c2-e63e8046d355@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 7FEFFC0007 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: 49h34uhjc7ttizbf6n3ushktbkpru64d X-HE-Tag: 1678936063-851125 X-HE-Meta: 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 E1azdzAG zV12BI7nspp3DgfRpKrf7O7dwfG3LO4OHKsy0GD2ekgIQX6uQSopo5roVfA3lFIkOeoe9eyd9TZwWqVNlaCutoeEyr2XhCinOaFEaKGGbi962sunbDkPnjFURBOllgFtLt9i7ijZhhn+64rHombRcM+lQy6vdea4IHROO9WuZzEemWsjrLqh76nXMJth2y0vnK8/csg2z/Ik4BUgSgzYBgoctuFCS4xaGZAh9ICefrIc9Pq0bzDKFzv9Ltjm/hVEPvbI1BACGNyE/OJYfNIdJkV+s3g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 15, 2023 at 05:25:49PM -0700, Yosry Ahmed wrote: [snipped 80 lines. please learn to trim] > I think instead of explicitly checking page->memcg_data, we can check > PageTail() and return explicitly for tail pages tails, check > PageSlab() to print the message for slab pages, then get the page's > memcg through folio_memcg_check(page_folio(page)). > > Something like: > > static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, > struct page *page) > { > ... > rcu_read_lock(); > > /* Only head pages hold refs to a memcg */ > if (PageTail(page)) > goto out_unlock; > > if (PageSlab(page)) > ret += scnprintf(kbuf + ret, count - ret, "Slab cache page\n"); > > memcg = folio_memcg_check(page_folio(page)); > if (!memcg) > goto out_unlock; > ... > } > > Matthew, What do you think? Brrr, this is hard. read_page_owner() holds no locks or references, so pages can transform between being head/tail/order-0 while we're running. It _tries_ to skip over tail pages in the most inefficient way possible: if (!IS_ALIGNED(pfn, 1 << page_owner->order)) goto ext_put_continue; But any attempt to use folio APIs is going to risk tripping the assertions in the folio code that it's not a tail. This requires more thought.