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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id z25sm1368942otj.68.2021.03.23.13.10.27 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Tue, 23 Mar 2021 13:10:28 -0700 (PDT) Date: Tue, 23 Mar 2021 13:10:13 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Johannes Weiner cc: Hugh Dickins , Andrew Morton , Matthew Wilcox , Michal Hocko , 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: 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: rt6whtdbdkqkkateotut35fjsc4p658a X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 85FBB4080F4A Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf26; identity=mailfrom; envelope-from=""; helo=mail-ot1-f50.google.com; client-ip=209.85.210.50 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616530229-492604 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 Tue, 23 Mar 2021, Johannes Weiner wrote: > From f6f062a3ec46f4fb083dcf6792fde9723f18cfc5 Mon Sep 17 00:00:00 2001 > From: Johannes Weiner > Date: Fri, 19 Mar 2021 02:17:00 -0400 > Subject: [PATCH] mm: page_alloc: fix allocation imbalances from speculative > cache lookup > > 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. > > There are currently two problems with it: > > 1. It checks PageHead() to see whether we're dealing with a compound > page after put_page_testzero(). But the speculative lookup could > have freed the page after our put and cleared PageHead, in which > case we would double free the tail pages. > > To fix this, test PageHead before the put and cache the result for > afterwards. > > 2. 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. > > To fix this, add a split_page_memcg() before it starts freeing tail > pages, to ensure they all have page->memcg set up. > > While at it, also update the comments a bit to clarify what exactly is > happening to the page during that race. > > Fixes: e320d3012d25 mm/page_alloc.c: fix freeing non-compound pages > Reported-by: Hugh Dickins > Reported-by: Matthew Wilcox > Signed-off-by: Johannes Weiner > Cc: # 5.10+ This is great, thanks Hannes. Acked-by: Hugh Dickins I know that 5.10-stable rejected the two split_page_memcg() patches: we shall need those in, I'll send GregKH the fixups, but not today. > --- > mm/page_alloc.c | 41 +++++++++++++++++++++++++++++++++++------ > 1 file changed, 35 insertions(+), 6 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index c53fe4fa10bf..8aab1e87fa3c 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 +5123,41 @@ 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)) > + bool compound = PageHead(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; > + } > + > + /* > + * Ok, the speculative reference will put and free the page. > + * > + * - If this was an order-0 page, we're done. > + * > + * - If the page was compound, the other side will free the > + * entire page and we're done here as well. Just note that > + * freeing clears PG_head, so it can only be read reliably > + * before the put_page_testzero(). > + * > + * - If the page was of higher order but NOT marked compound, > + * the other side will know nothing about our buddy pages > + * and only free the order-0 page at the start of our block. > + * 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 (order > 0 && !compound) { > + split_page_memcg(page, 1 << order); > while (order-- > 0) > free_the_page(page + (1 << order), order); > + } > } > EXPORT_SYMBOL(__free_pages); > > -- > 2.31.0