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McKenney" , Andrew Morton , linux-mm , LKML , John Dias , David Hildenbrand Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page Message-ID: References: <8f083802-7ab0-15ec-b37d-bc9471eea0b1@nvidia.com> <20220511234534.GG1790663@paulmck-ThinkPad-P17-Gen-1> <0d90390c-3624-4f93-f8bd-fb29e92237d3@nvidia.com> <20220512002207.GJ1790663@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: jzcado31j9pe8fwa6zgf89k7dhstuhtd X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: AE136400A8 Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Z7Xz3sS2; spf=pass (imf27.hostedemail.com: domain of minchan.kim@gmail.com designates 209.85.215.178 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com; dmarc=fail reason="SPF not aligned (relaxed), DKIM not aligned (relaxed)" header.from=kernel.org (policy=none) X-Rspam-User: X-HE-Tag: 1652317435-339649 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, May 11, 2022 at 05:34:52PM -0700, John Hubbard wrote: > On 5/11/22 17:26, Minchan Kim wrote: > > > > Let me try to say this more clearly: I don't think that the following > > > > __READ_ONCE() statement can actually help anything, given that > > > > get_pageblock_migratetype() is non-inlined: > > > > > > > > + int __mt = get_pageblock_migratetype(page); > > > > + int mt = __READ_ONCE(__mt); > > > > + > > > > + if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE)) > > > > + return false; > > > > > > > > > > > > Am I missing anything here? > > > > > > In the absence of future aggression from link-time optimizations (LTO), > > > you are missing nothing. > > > > A thing I want to note is Android kernel uses LTO full mode. > > Thanks Paul for explaining the state of things. > > Minchan, how about something like very close to your original draft, > then, but with a little note, and the "&" as well: > > int __mt = get_pageblock_migratetype(page); > > /* > * Defend against future compiler LTO features, or code refactoring > * that inlines the above function, by forcing a single read. Because, this > * routine races with set_pageblock_migratetype(), and we want to avoid > * reading zero, when actually one or the other flags was set. > */ > int mt = __READ_ONCE(__mt); > > if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE)) > return false; > > > ...which should make everyone comfortable and protected from the > future sins of the compiler and linker teams? :) Will take. Thanks.