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[209.132.183.28]) by mx.google.com with ESMTPS id s21si1218636qki.43.2019.01.30.07.04.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Jan 2019 07:04:45 -0800 (PST) Received-SPF: pass (google.com: domain of fweimer@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of fweimer@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=fweimer@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1761D155E0; Wed, 30 Jan 2019 15:04:43 +0000 (UTC) Received: from oldenburg2.str.redhat.com (dhcp-192-219.str.redhat.com [10.33.192.219]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D198D100164E; Wed, 30 Jan 2019 15:04:37 +0000 (UTC) From: Florian Weimer To: Vlastimil Babka Cc: Andrew Morton , Linus Torvalds , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Peter Zijlstra , Greg KH , Jann Horn , Jiri Kosina , Dominique Martinet , Andy Lutomirski , Dave Chinner , Kevin Easton , Matthew Wilcox , Cyril Hrubis , Tejun Heo , "Kirill A . Shutemov" , Daniel Gruss , Jiri Kosina Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O References: <20190130124420.1834-1-vbabka@suse.cz> <20190130124420.1834-3-vbabka@suse.cz> Date: Wed, 30 Jan 2019 16:04:36 +0100 In-Reply-To: <20190130124420.1834-3-vbabka@suse.cz> (Vlastimil Babka's message of "Wed, 30 Jan 2019 13:44:19 +0100") Message-ID: <87munii3uj.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 30 Jan 2019 15:04:44 +0000 (UTC) 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: * Vlastimil Babka: > 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. Thanks, Florian