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McKenney" , John Dias , David Hildenbrand Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page Message-ID: References: <20220510211743.95831-1-minchan@kernel.org> <857d21da-5de2-fa3e-b1ce-41cc1cfb0191@nvidia.com> <2ffa7670-04ea-bb28-28f8-93a9b9eea7e8@nvidia.com> <54b5d177-f2f4-cef2-3a68-cd3b0b276f86@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 5EC29160089 X-Stat-Signature: iadnanqocd93atszeugga1i11ojmhbk5 X-Rspam-User: Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Ej7Rha+d; spf=pass (imf08.hostedemail.com: domain of minchan.kim@gmail.com designates 209.85.210.178 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-HE-Tag: 1652310525-608973 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, May 11, 2022 at 03:49:06PM -0700, John Hubbard wrote: > On 5/11/22 15:37, Minchan Kim wrote: > > > Yes. But one thing that is still unanswered, that I think you can > > > answer, is: even if the compiler *did* re-read the mt variable, what > > > problems could that cause? I claim "no problems", because there is > > > no combination of 0, _CMA, _ISOLATE, _CMA|ISOLATE that will cause > > > problems here. > > > > What scenario I am concerning with __READ_ONCE so compiler > > inlining get_pageblock_migratetype two times are > > > > CPU 0 CPU 1 > > alloc_contig_range > > is_pinnable_page start_isolate_page_range > > set_pageblock_migratetype(MIGRATE_ISOLATE) > > if (get_pageeblock_migratetype(page) == MIGRATE_CMA) > > so it's false > > undo: > > set_pageblock_migratetype(MIGRATE_CMA) > > if (get_pageeblock_migratetype(page) == MIGRATE_ISOLATE) > > so it's false > > > > In the end, CMA memory would be pinned by CPU 0 process > > so CMA allocation keep failed until the process release the > > refcount. > > > > OK, so the code checks the wrong item each time. But the code really > only needs to know "is either _CMA or _ISOLATE set?". And so you Yes. > can just sidestep the entire question by writing it like this: > > int mt = get_pageblock_migratetype(page); > > if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA)) > return false; I am confused. Isn't it same question? set_pageblock_migratetype(MIGRATE_ISOLATE) if (get_pageblock_migrate(page) & MIGRATE_CMA) set_pageblock_migratetype(MIGRATE_CMA) if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)