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[198.145.29.99]) by mx.google.com with ESMTPS id l10si13747254pgp.411.2019.07.22.17.52.31 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Jul 2019 17:52:31 -0700 (PDT) Received-SPF: pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) client-ip=198.145.29.99; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=default header.b=qldAgseH; spf=pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 47D7B2199C; Tue, 23 Jul 2019 00:52:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563843151; bh=QoDXFktQWBBjpdyHJklib+lJgS4BW2A6BFE+HFerPWQ=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=qldAgseHrPTRQybgwoDYuxEyG8jbKlaOwXXZCKp00RlNMoCOAGqSm4bAf+McROJz/ mfL6Is692vmHZ+Bag8epiebNzp9yZjUrQw5ExCZEV6QC2Eng/d11n0S42hba7gjy8G myaupLK4kQRgZBzodyvJKQEr/bHTIZpHrw5upEQ0= Date: Mon, 22 Jul 2019 17:52:30 -0700 From: Andrew Morton To: Konstantin Khlebnikov Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe , Johannes Weiner , linux-fsdevel@vger.kernel.org, Jan Kara Subject: Re: [PATCH 1/2] mm/filemap: don't initiate writeback if mapping has no dirty pages Message-Id: <20190722175230.d357d52c3e86dc87efbd4243@linux-foundation.org> In-Reply-To: <156378816804.1087.8607636317907921438.stgit@buzz> References: <156378816804.1087.8607636317907921438.stgit@buzz> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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: (cc linux-fsdevel and Jan) On Mon, 22 Jul 2019 12:36:08 +0300 Konstantin Khlebnikov wrote: > Functions like filemap_write_and_wait_range() should do nothing if inode > has no dirty pages or pages currently under writeback. But they anyway > construct struct writeback_control and this does some atomic operations > if CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and > updates state of writeback ownership, on slow path might be more work. > Current this path is safely avoided only when inode mapping has no pages. > > For example generic_file_read_iter() calls filemap_write_and_wait_range() > at each O_DIRECT read - pretty hot path. > > This patch skips starting new writeback if mapping has no dirty tags set. > If writeback is already in progress filemap_write_and_wait_range() will > wait for it. > > ... > > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -408,7 +408,8 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start, > .range_end = end, > }; > > - if (!mapping_cap_writeback_dirty(mapping)) > + if (!mapping_cap_writeback_dirty(mapping) || > + !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) > return 0; > > wbc_attach_fdatawrite_inode(&wbc, mapping->host); How does this play with tagged_writepages? We assume that no tagging has been performed by any __filemap_fdatawrite_range() caller?