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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id k4sm5877496qke.13.2021.03.19.18.53.00 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Fri, 19 Mar 2021 18:53:01 -0700 (PDT) Date: Fri, 19 Mar 2021 18:52:58 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Johannes Weiner cc: Andrew Morton , Matthew Wilcox , Michal Hocko , Hugh Dickins , Zhou Guanghui , Zi Yan , Shakeel Butt , Roman Gushchin , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: page_alloc: fix memcg accounting leak in speculative cache lookup In-Reply-To: <20210319071547.60973-1-hannes@cmpxchg.org> Message-ID: References: <20210319071547.60973-1-hannes@cmpxchg.org> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Stat-Signature: kqdjz373pyuir53ox3sffaai6d51q4kg X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: E546EE0011C5 Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf21; identity=mailfrom; envelope-from=""; helo=mail-qt1-f182.google.com; client-ip=209.85.160.182 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616205182-956916 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 Fri, 19 Mar 2021, Johannes Weiner wrote: > When the freeing of a higher-order page block (non-compound) races > with a speculative page cache lookup, __free_pages() needs to leave > the first order-0 page in the chunk to the lookup but free the buddy > pages that the lookup doesn't know about separately. > > However, if such a higher-order page is charged to a memcg (e.g. !vmap > kernel stack)), only the first page of the block has page->memcg > set. That means we'll uncharge only one order-0 page from the entire > block, and leak the remainder. > > Add a split_page_memcg() to __free_pages() right before it starts > taking the higher-order page apart and freeing its individual > constituent pages. This ensures all of them will have the memcg > linkage set up for correct uncharging. Also update the comments a bit > to clarify what exactly is happening to the page during that race. > > This bug is old and has its roots in the speculative page cache patch > and adding cgroup accounting of kernel pages. There are no known user > reports. A backport to stable is therefor not warranted. > > Reported-by: Matthew Wilcox > Signed-off-by: Johannes Weiner Acked-by: Hugh Dickins to the split_page_memcg() addition etc, but a doubt just hit me on the original e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages"): see comment below. > --- > mm/page_alloc.c | 33 +++++++++++++++++++++++++++------ > 1 file changed, 27 insertions(+), 6 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index c53fe4fa10bf..f4bd56656402 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -5112,10 +5112,9 @@ static inline void free_the_page(struct page *page, unsigned int order) > * the allocation, so it is easy to leak memory. Freeing more memory > * than was allocated will probably emit a warning. > * > - * If the last reference to this page is speculative, it will be released > - * by put_page() which only frees the first page of a non-compound > - * allocation. To prevent the remaining pages from being leaked, we free > - * the subsequent pages here. If you want to use the page's reference > + * This function isn't a put_page(). Don't let the put_page_testzero() > + * fool you, it's only to deal with speculative cache references. It > + * WILL free pages directly. If you want to use the page's reference > * count to decide when to free the allocation, you should allocate a > * compound page, and use put_page() instead of __free_pages(). > * > @@ -5124,11 +5123,33 @@ static inline void free_the_page(struct page *page, unsigned int order) > */ > void __free_pages(struct page *page, unsigned int order) > { > - if (put_page_testzero(page)) > + /* > + * 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); > - else if (!PageHead(page)) > + return; > + } > + > + /* > + * The speculative reference will put and free the page. > + * > + * However, if the speculation was into a higher-order page > + * chunk 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) { The put_page_testzero() has released our reference to the first subpage of page: it's now under the control of the racing speculative lookup. So it seems to me unsafe to be checking PageHead(page) here: if it was actually a compound page, PageHead might already be cleared by now, and we doubly free its tail pages below? I think we need to use a "bool compound = PageHead(page)" on entry to __free_pages(). Or alternatively, it's wrong to call __free_pages() on a compound page anyway, so we should not check PageHead at all, except in a WARN_ON_ONCE(PageCompound(page)) at the start? And would it be wrong to fix that too in this patch? Though it ought then to be backported to 5.10 stable. > + split_page_memcg(page, 1 << order); > while (order-- > 0) > free_the_page(page + (1 << order), order); > + } > } > EXPORT_SYMBOL(__free_pages); > > -- > 2.30.1