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[142.162.115.133]) by smtp.gmail.com with ESMTPSA id j7sm12802577qtb.87.2021.01.19.10.47.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Jan 2021 10:47:52 -0800 (PST) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1l1w2h-003q3W-Vv; Tue, 19 Jan 2021 14:47:51 -0400 Date: Tue, 19 Jan 2021 14:47:51 -0400 From: Jason Gunthorpe To: Pavel Tatashin Cc: LKML , linux-mm , Andrew Morton , Vlastimil Babka , Michal Hocko , David Hildenbrand , Oscar Salvador , Dan Williams , Sasha Levin , Tyler Hicks , Joonsoo Kim , mike.kravetz@oracle.com, Steven Rostedt , Ingo Molnar , Peter Zijlstra , Mel Gorman , Matthew Wilcox , David Rientjes , John Hubbard , Linux Doc Mailing List , Ira Weiny , linux-kselftest@vger.kernel.org Subject: Re: [PATCH v5 08/14] mm/gup: do not allow zero page for pinned pages Message-ID: <20210119184751.GD4605@ziepe.ca> References: <20210119043920.155044-1-pasha.tatashin@soleen.com> <20210119043920.155044-9-pasha.tatashin@soleen.com> <20210119183013.GB4605@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On Tue, Jan 19, 2021 at 01:34:26PM -0500, Pavel Tatashin wrote: > On Tue, Jan 19, 2021 at 1:30 PM Jason Gunthorpe wrote: > > > > On Mon, Jan 18, 2021 at 11:39:14PM -0500, Pavel Tatashin wrote: > > > Zero page should not be used for long term pinned pages. Once pages > > > are pinned their physical addresses cannot changed until they are unpinned. > > > > > > Guarantee to always return real pages when they are pinned by adding > > > FOLL_WRITE. > > > > > > Signed-off-by: Pavel Tatashin > > > mm/gup.c | 10 +++++++++- > > > 1 file changed, 9 insertions(+), 1 deletion(-) > > > > No, this will definitely break things > > What will break Things assuming GUP doesn't break COW, making all GUP WRITE was already tried and revered for some other reason > > Why does the zero page have to be movable? > > It is not even about being movable, we can't cow pinned pages returned > by GUP call, how can we use zero page for that? The zero page is always zero, it is never written to. What does cow matter? Jason