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[195.135.220.15]) by mx.google.com with ESMTPS id d8-v6si968994ejy.275.2019.01.30.07.15.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Jan 2019 07:15:59 -0800 (PST) Received-SPF: softfail (google.com: domain of transitioning jikos@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning jikos@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=jikos@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 74A5AAF1C; Wed, 30 Jan 2019 15:15:58 +0000 (UTC) Date: Wed, 30 Jan 2019 16:15:55 +0100 (CET) From: Jiri Kosina To: Florian Weimer cc: Vlastimil Babka , Andrew Morton , Linus Torvalds , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Peter Zijlstra , Greg KH , Jann Horn , Dominique Martinet , Andy Lutomirski , Dave Chinner , Kevin Easton , Matthew Wilcox , Cyril Hrubis , Tejun Heo , "Kirill A . Shutemov" , Daniel Gruss Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O In-Reply-To: <87munii3uj.fsf@oldenburg2.str.redhat.com> Message-ID: References: <20190130124420.1834-1-vbabka@suse.cz> <20190130124420.1834-3-vbabka@suse.cz> <87munii3uj.fsf@oldenburg2.str.redhat.com> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII 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 Wed, 30 Jan 2019, Florian Weimer wrote: > > preadv2(RWF_NOWAIT) can be used to open a side-channel to pagecache > > contents, as it reveals metadata about residency of pages in > > pagecache. > > > > If preadv2(RWF_NOWAIT) returns immediately, it provides a clear "page > > not resident" information, and vice versa. > > > > Close that sidechannel by always initiating readahead on the cache if > > we encounter a cache miss for preadv2(RWF_NOWAIT); with that in place, > > probing the pagecache residency itself will actually populate the > > cache, making the sidechannel useless. > > I think this needs to use a different flag because the semantics are so > much different. If I understand this change correctly, previously, > RWF_NOWAIT essentially avoided any I/O, and now it does not. It still avoid synchronous I/O, due to this code still being in place: if (!PageUptodate(page)) { if (iocb->ki_flags & IOCB_NOWAIT) { put_page(page); goto would_block; } but goes the would_block path only after initiating asynchronous readahead. -- Jiri Kosina SUSE Labs