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[35.231.104.69]) by smtp.gmail.com with ESMTPSA id s74sm308968vkb.48.2020.04.30.15.23.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Apr 2020 15:23:48 -0700 (PDT) Date: Thu, 30 Apr 2020 22:23:47 +0000 From: Dennis Zhou To: Andrew Morton Cc: fdmanana@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, dennis@kernel.org, tj@kernel.org, cl@linux.com, linux-btrfs@vger.kernel.org, Filipe Manana Subject: Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context Message-ID: <20200430222347.GA164259@google.com> References: <20200430164356.15543-1-fdmanana@kernel.org> <20200430144018.c855f031b321d68e5c89b30c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200430144018.c855f031b321d68e5c89b30c@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 30, 2020 at 02:40:18PM -0700, Andrew Morton wrote: > On Thu, 30 Apr 2020 17:43:56 +0100 fdmanana@kernel.org wrote: > > > From: Filipe Manana > > > > Since 5.7-rc1, on btrfs we have a percpu counter initialization for which > > we always pass a GFP_KERNEL gfp_t argument (this happens since commit > > 2992df73268f78 ("btrfs: Implement DREW lock")). That is safe in some > > contextes but not on others where allowing fs reclaim could lead to a > > deadlock because we are either holding some btrfs lock needed for a > > transaction commit or holding a btrfs transaction handle open. Because > > of that we surround the call to the function that initializes the percpu > > counter with a NOFS context using memalloc_nofs_save() (this is done at > > btrfs_init_fs_root()). > > > > However it turns out that this is not enough to prevent a possible > > deadlock because percpu_alloc() determines if it is in an atomic context > > by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this > > case) and it is not aware that a NOFS context is set. Because it thinks > > it is in a non atomic context it locks the pcpu_alloc_mutex, which can > > result in a btrfs deadlock when pcpu_balance_workfn() is running, has > > acquired that mutex and is waiting for reclaim, while the btrfs task that > > called percpu_counter_init() (and therefore percpu_alloc()) is holding > > either the btrfs commit_root semaphore or a transaction handle (done at > > fs/btrfs/backref.c:iterate_extent_inodes()), which prevents reclaim from > > finishing as an attempt to commit the current btrfs transaction will > > deadlock. > > > > Patch looks good and seems sensible, thanks. > Acked-by: Dennis Zhou > But why did btrfs use memalloc_nofs_save()/restore() rather than > s/GFP_KERNEL/GFP_NOFS/? I would also like to know. Thanks, Dennis