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Date: Mon, 4 Feb 2019 16:08:02 +0000 From: Christopher Lameter <cl@...ux.com> To: john.hubbard@...il.com cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, Al Viro <viro@...iv.linux.org.uk>, Christian Benvenuti <benve@...co.com>, Christoph Hellwig <hch@...radead.org>, Dan Williams <dan.j.williams@...el.com>, Dave Chinner <david@...morbit.com>, Dennis Dalessandro <dennis.dalessandro@...el.com>, Doug Ledford <dledford@...hat.com>, Jan Kara <jack@...e.cz>, Jason Gunthorpe <jgg@...pe.ca>, Jerome Glisse <jglisse@...hat.com>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...nel.org>, Mike Rapoport <rppt@...ux.ibm.com>, Mike Marciniszyn <mike.marciniszyn@...el.com>, Ralph Campbell <rcampbell@...dia.com>, Tom Talpey <tom@...pey.com>, LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com> Subject: Re: [PATCH 0/6] RFC v2: mm: gup/dma tracking On Sun, 3 Feb 2019, john.hubbard@...il.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.
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