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[195.135.220.15]) by mx.google.com with ESMTPS id f1si31385673ejb.158.2019.08.08.04.09.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 08 Aug 2019 04:09:22 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz 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 8B21DAE16; Thu, 8 Aug 2019 11:09:21 +0000 (UTC) Subject: Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*() To: Michal Hocko , John Hubbard Cc: john.hubbard@gmail.com, Andrew Morton , Christoph Hellwig , Ira Weiny , Jan Kara , Jason Gunthorpe , Jerome Glisse , LKML , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Dan Williams , Daniel Black , Matthew Wilcox , Mike Kravetz References: <20190805222019.28592-1-jhubbard@nvidia.com> <20190805222019.28592-2-jhubbard@nvidia.com> <20190807110147.GT11812@dhcp22.suse.cz> <01b5ed91-a8f7-6b36-a068-31870c05aad6@nvidia.com> <20190808062155.GF11812@dhcp22.suse.cz> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190808062155.GF11812@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 8/8/19 8:21 AM, Michal Hocko wrote: > On Wed 07-08-19 16:32:08, John Hubbard wrote: >> On 8/7/19 4:01 AM, Michal Hocko wrote: >>> On Mon 05-08-19 15:20:17, john.hubbard@gmail.com wrote: >>>> From: John Hubbard >>>> >>>> For pages that were retained via get_user_pages*(), release those pages >>>> via the new put_user_page*() routines, instead of via put_page() or >>>> release_pages(). >>> >>> Hmm, this is an interesting code path. There seems to be a mix of pages >>> in the game. We get one page via follow_page_mask but then other pages >>> in the range are filled by __munlock_pagevec_fill and that does a direct >>> pte walk. Is using put_user_page correct in this case? Could you explain >>> why in the changelog? >>> >> >> Actually, I think follow_page_mask() gets all the pages, right? And the >> get_page() in __munlock_pagevec_fill() is there to allow a pagevec_release() >> later. > > Maybe I am misreading the code (looking at Linus tree) but munlock_vma_pages_range > calls follow_page for the start address and then if not THP tries to > fill up the pagevec with few more pages (up to end), do the shortcut > via manual pte walk as an optimization and use generic get_page there. That's true. However, I'm not sure munlocking is where the put_user_page() machinery is intended to be used anyway? These are short-term pins for struct page manipulation, not e.g. dirtying of page contents. Reading commit fc1d8e7cca2d I don't think this case falls within the reasoning there. Perhaps not all GUP users should be converted to the planned separate GUP tracking, and instead we should have a GUP/follow_page_mask() variant that keeps using get_page/put_page?