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[184.145.4.219]) by smtp.gmail.com with ESMTPSA id 7sm15927040qtu.38.2021.05.18.16.45.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 May 2021 16:45:06 -0700 (PDT) Date: Tue, 18 May 2021 19:45:05 -0400 From: Peter Xu To: Jason Gunthorpe Subject: Re: [PATCH v8 5/8] mm: Device exclusive memory access Message-ID: References: <20210407084238.20443-1-apopple@nvidia.com> <20210407084238.20443-6-apopple@nvidia.com> <47694715.suB6H4Uo8R@nvdebian> <20210518173334.GE1002214@nvidia.com> <20210518194509.GF1002214@nvidia.com> <20210518230327.GG1002214@nvidia.com> MIME-Version: 1.0 In-Reply-To: <20210518230327.GG1002214@nvidia.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rcampbell@nvidia.com, willy@infradead.org, linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, bsingharora@gmail.com, Alistair Popple , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, hch@infradead.org, linux-mm@kvack.org, jglisse@redhat.com, bskeggs@redhat.com, jhubbard@nvidia.com, akpm@linux-foundation.org, Christoph Hellwig Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Tue, May 18, 2021 at 08:03:27PM -0300, Jason Gunthorpe wrote: > Logically during fork all these device exclusive pages should be > reverted back to their CPU pages, write protected and the CPU page PTE > copied to the fork. > > We should not copy the device exclusive page PTE to the fork. I think > I pointed to this on an earlier rev.. Agreed. Though please see the question I posted in the other thread: now I am not very sure whether we'll be able to mark a page as device exclusive if that page has mapcount>1. > > We can optimize this into the various variants above, but logically > device exclusive stop existing during fork. Makes sense, I think that's indeed what this patch did at least for the COW case, so I think Alistair did address that comment. It's just that I think we need to drop the other !COW case (imho that should correspond to the changes in copy_nonpresent_pte()) in this patch to guarantee it. I also hope we don't make copy_pte_range() even more complicated just to do the lock_page() right, so we could fail the fork() if the lock is hard to take. -- Peter Xu