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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id f18sm1787318qko.34.2021.12.10.11.19.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Dec 2021 11:19:12 -0800 (PST) Date: Fri, 10 Dec 2021 11:18:59 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Andrew Morton cc: Hugh Dickins , Matthew Wilcox , Vlastimil Babka , William Kucharski , Christoph Hellwig , "Kirill A. Shutemov" , Mike Rapoport , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative() In-Reply-To: <20211210092003.cf84354b406a47253afc868c@linux-foundation.org> Message-ID: <7e783f99-148f-c7aa-08a-4c4c45c5506a@google.com> References: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> <20211210092003.cf84354b406a47253afc868c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Stat-Signature: nwmb4eomqxygqrsaskzzu4w77pgcckb3 Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=iPHEafCo; spf=pass (imf20.hostedemail.com: domain of hughd@google.com designates 209.85.160.172 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: EEB941C0010 X-HE-Tag: 1639163952-218132 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, 10 Dec 2021, Andrew Morton wrote: > On Wed, 8 Dec 2021 23:19:18 -0800 (PST) Hugh Dickins wrote: > > > It is not easily reproducible, but on 5.16-rc I have several times hit > > the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative(): > > usually from filemap_get_read_batch() for an ext4 read, yesterday from > > next_uptodate_page() from filemap_map_pages() for a shmem fault. > > > > That BUG used to be placed where page_ref_add_unless() had succeeded, > > but now it is placed before folio_ref_add_unless() is attempted: that > > is not safe, since it is only the acquired reference which makes the > > page safe from racing THP collapse or split. > > > > We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu() > > has succeeded; but I don't think it adds much value - just delete it. > > > > Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()") > > Signed-off-by: Hugh Dickins > > I added cc:stable to this. Thanks, but no, cc:stable not needed: the fixed commit went into 5.16-rc1, and did not go to stable itself. There was an identical VM_BUG_ON_PAGE in the old __page_cache_add_speculative(), but that one was correctly placed, so there's no need for the old one to be removed. Hugh