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Mon, 25 May 2026 13:13:36 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id JXF/NgBLFGo7ZgAAD6G6ig (envelope-from ); Mon, 25 May 2026 13:13:36 +0000 Date: Mon, 25 May 2026 15:13:31 +0200 From: David Sterba To: Cen Zhang Cc: clm@fb.com, dsterba@suse.com, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com Subject: Re: [PATCH] btrfs: annotate lockless read of defrag_bytes in should_nocow() Message-ID: <20260525131331.GD12792@twin.jikos.cz> Reply-To: dsterba@suse.cz References: <20260401022153.743576-1-zzzccc427@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260401022153.743576-1-zzzccc427@gmail.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) X-Spam-Score: -4.00 X-Spamd-Result: default: False [-4.00 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; HAS_REPLYTO(0.30)[dsterba@suse.cz]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MIME_TRACE(0.00)[0:+]; TO_DN_SOME(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; FREEMAIL_TO(0.00)[gmail.com]; RCVD_TLS_ALL(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; ARC_NA(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; RCPT_COUNT_FIVE(0.00)[6]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[fb.com,suse.com,vger.kernel.org,gmail.com]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; REPLYTO_ADDR_EQ_FROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[twin.jikos.cz:mid,suse.cz:replyto,imap1.dmz-prg2.suse.org:helo]; RCVD_COUNT_TWO(0.00)[2]; REPLYTO_DOM_NEQ_TO_DOM(0.00)[] X-Spam-Level: X-Spam-Flag: NO On Wed, Apr 01, 2026 at 10:21:53AM +0800, Cen Zhang wrote: > should_nocow() reads inode->defrag_bytes without holding inode->lock, > while btrfs_set_delalloc_extent() and btrfs_clear_delalloc_extent() > update it under that spinlock. > > This is a data race. The read is a quick check used to decide whether > to fall back to COW for a NOCOW inode: if defrag_bytes is non-zero and > the range is tagged EXTENT_DEFRAG, we force COW so that defragmentation > can rewrite the extent. Reading a stale value is harmless because: > > - A missed increment may skip COW once, but the defrag pass will > redo the extent later. > - A stale non-zero may force an unnecessary COW, which is a minor > efficiency loss, not a correctness issue. > > On 64-bit platforms an aligned u64 load is naturally atomic so tearing > cannot happen. On 32-bit platforms u64 may tear, but we only test for > zero vs non-zero, so the heuristic stays correct regardless. > > Add READ_ONCE() to prevent the compiler from caching or splitting the > load and to document the intentional lock-free pattern. > > Fixes: 47059d930f0e ("Btrfs: make defragment work with nodatacow option") > Signed-off-by: Cen Zhang Added to for-next, with the data_race() annotation. Thanks.