From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DC329C282D7 for ; Wed, 30 Jan 2019 15:04:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B2BB320989 for ; Wed, 30 Jan 2019 15:04:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731377AbfA3PEp (ORCPT ); Wed, 30 Jan 2019 10:04:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:60616 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727162AbfA3PEo (ORCPT ); Wed, 30 Jan 2019 10:04:44 -0500 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) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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