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[54.240.9.54]) by mx.google.com with ESMTPS id g89si5567310qtd.118.2019.02.04.08.08.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 04 Feb 2019 08:08:03 -0800 (PST) Received-SPF: pass (google.com: domain of 01000168b94439c0-28d5e096-718d-4e39-a7af-20d0a6d7b768-000000@amazonses.com designates 54.240.9.54 as permitted sender) client-ip=54.240.9.54; Authentication-Results: mx.google.com; dkim=pass header.i=@amazonses.com header.s=ug7nbtf4gccmlpwj322ax3p6ow6yfsug header.b="QUHV/hav"; spf=pass (google.com: domain of 01000168b94439c0-28d5e096-718d-4e39-a7af-20d0a6d7b768-000000@amazonses.com designates 54.240.9.54 as permitted sender) smtp.mailfrom=01000168b94439c0-28d5e096-718d-4e39-a7af-20d0a6d7b768-000000@amazonses.com DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/simple; s=ug7nbtf4gccmlpwj322ax3p6ow6yfsug; d=amazonses.com; t=1549296482; h=Date:From:To:cc:Subject:In-Reply-To:Message-ID:References:MIME-Version:Content-Type:Feedback-ID; bh=Li4m9VgzDn107uj2skKg4pEfVvnsw/uPImCizgtRdaU=; b=QUHV/hav0zNxHS+Wl87A0BoUBsD74FYkkpUunFx1l6GFEgByPtSLcygI9/Pk2syi C6bDLjL1Pdfq52boznJ/KtH8hLI32bTAvzHqB85YGHmpczLAcFacjAUZoDP3w0btUvN y/IW6A2TwMWHYVnMkaimoIdQsZdq3gV6plhPWU+k= Date: Mon, 4 Feb 2019 16:08:02 +0000 From: Christopher Lameter X-X-Sender: cl@nuc-kabylake To: john.hubbard@gmail.com cc: Andrew Morton , linux-mm@kvack.org, Al Viro , Christian Benvenuti , Christoph Hellwig , Dan Williams , Dave Chinner , Dennis Dalessandro , Doug Ledford , Jan Kara , Jason Gunthorpe , Jerome Glisse , Matthew Wilcox , Michal Hocko , Mike Rapoport , Mike Marciniszyn , Ralph Campbell , Tom Talpey , LKML , linux-fsdevel@vger.kernel.org, John Hubbard Subject: Re: [PATCH 0/6] RFC v2: mm: gup/dma tracking In-Reply-To: <20190204052135.25784-1-jhubbard@nvidia.com> Message-ID: <01000168b94439c0-28d5e096-718d-4e39-a7af-20d0a6d7b768-000000@email.amazonses.com> References: <20190204052135.25784-1-jhubbard@nvidia.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-SES-Outgoing: 2019.02.04-54.240.9.54 Feedback-ID: 1.us-east-1.fQZZZ0Xtj2+TD7V5apTT/NrT6QKuPgzCT/IC7XYgDKI=:AmazonSES 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 Sun, 3 Feb 2019, john.hubbard@gmail.com wrote: > Some kernel components (file systems, device drivers) need to access > memory that is specified via process virtual address. For a long time, the > API to achieve that was get_user_pages ("GUP") and its variations. However, > GUP has critical limitations that have been overlooked; in particular, GUP > does not interact correctly with filesystems in all situations. That means > that file-backed memory + GUP is a recipe for potential problems, some of > which have already occurred in the field. It may be worth noting a couple of times in this text that this was designed for anonymous memory and that such use is/was ok. We are talking about a use case here using mmapped access with a regular filesystem that was not initially intended. The mmapping of from the hugepages filesystem is special in that it is not a device that is actually writing things back. Any use with a filesystem that actually writes data back to a medium is something that is broken.