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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id f27sm2692401qkh.118.2021.03.11.12.37.33 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Thu, 11 Mar 2021 12:37:34 -0800 (PST) Date: Thu, 11 Mar 2021 12:37:20 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: Johannes Weiner , Matthew Wilcox , Zhou Guanghui , linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, hughd@google.com, kirill.shutemov@linux.intel.com, npiggin@gmail.com, ziy@nvidia.com, wangkefeng.wang@huawei.com, guohanjun@huawei.com, dingtianhong@huawei.com, chenweilong@huawei.com, rui.xiang@huawei.com Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page In-Reply-To: Message-ID: References: <20210304074053.65527-1-zhouguanghui1@huawei.com> <20210304074053.65527-3-zhouguanghui1@huawei.com> <20210308210225.GF3479805@casper.infradead.org> <20210309123255.GI3479805@casper.infradead.org> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 11 Mar 2021, Michal Hocko wrote: > On Thu 11-03-21 10:21:39, Johannes Weiner wrote: > > On Thu, Mar 11, 2021 at 09:37:02AM +0100, Michal Hocko wrote: > > > Johannes, Hugh, > > > > > > what do you think about this approach? If we want to stick with > > > split_page approach then we need to update the missing place Matthew has > > > pointed out. > > > > I find the __free_pages() code quite tricky as well. But for that > > reason I would actually prefer to initiate the splitting in there, > > since that's the place where we actually split the page, rather than > > spread the handling of this situation further out. > > > > The race condition shouldn't be hot, so I don't think we need to be as > > efficient about setting page->memcg_data only on the higher-order > > buddies as in Willy's scratch patch. We can call split_page_memcg(), > > which IMO should actually help document what's happening to the page. > > > > I think that function could also benefit a bit more from step-by-step > > documentation about what's going on. The kerneldoc is helpful, but I > > don't think it does justice to how tricky this race condition is. > > > > Something like this? > > > > void __free_pages(struct page *page, unsigned int order) > > { > > /* > > * Drop the base reference from __alloc_pages and free. In > > * case there is an outstanding speculative reference, from > > * e.g. the page cache, it will put and free the page later. > > */ > > if (likely(put_page_testzero(page))) { > > free_the_page(page, order); > > return; > > } > > > > /* > > * The speculative reference will put and free the page. > > * > > * However, if the speculation was into a higher-order page > > * that isn't marked compound, the other side will know > > * nothing about our buddy pages and only free the order-0 > > * page at the start of our chunk! We must split off and free > > * the buddy pages here. > > * > > * The buddy pages aren't individually refcounted, so they > > * can't have any pending speculative references themselves. > > */ > > if (!PageHead(page) && order > 0) { > > split_page_memcg(page, 1 << order); > > while (order-- > 0) > > free_the_page(page + (1 << order), order); > > } > > } > > Fine with me. Mathew was concerned about more places that do something > similar but I would say that if we find out more places we might > reconsider and currently stay with a reasonably clear model that it is > only head patch that carries the memcg information and split_page_memcg > is necessary to break such page into smaller pieces. I agree: I do like Johannes' suggestion best, now that we already have split_page_memcg(). Not too worried about contrived use of free_unref_page() here; and whether non-compound high-order pages should be perpetuated is a different discussion. Hugh