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Mon, 22 Jun 2020 17:02:32 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.93) (envelope-from ) id 1jnWOV-00CHMM-GL; Mon, 22 Jun 2020 21:02:31 -0300 Date: Mon, 22 Jun 2020 21:02:31 -0300 From: Jason Gunthorpe To: Jerome Glisse Subject: Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations Message-ID: <20200623000231.GW6578@ziepe.ca> References: <20200619151551.GP6578@ziepe.ca> <20200619172308.GQ6578@ziepe.ca> <20200619180935.GA10009@redhat.com> <20200619181849.GR6578@ziepe.ca> <56008d64-772d-5757-6136-f20591ef71d2@amd.com> <20200619195538.GT6578@ziepe.ca> <20200619203147.GC13117@redhat.com> <20200622114617.GU6578@ziepe.ca> <20200622201540.GB9708@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200622201540.GB9708@redhat.com> X-Mailman-Approved-At: Tue, 23 Jun 2020 07:49:16 +0000 X-BeenThere: amd-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Discussion list for AMD gfx List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-rdma , Felix Kuehling , Maarten Lankhorst , LKML , DRI Development , Christian =?utf-8?B?S8O2bmln?= , "moderated list:DMA BUFFER SHARING FRAMEWORK" , Thomas Hellstrom , amd-gfx list , Daniel Vetter , Daniel Vetter , "open list:DMA BUFFER SHARING FRAMEWORK" , Intel Graphics Development , Thomas =?utf-8?B?SGVsbHN0csO2bSAoSW50ZWwp?= , Mika Kuoppala Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: amd-gfx-bounces@lists.freedesktop.org Sender: "amd-gfx" On Mon, Jun 22, 2020 at 04:15:40PM -0400, Jerome Glisse wrote: > On Mon, Jun 22, 2020 at 08:46:17AM -0300, Jason Gunthorpe wrote: > > On Fri, Jun 19, 2020 at 04:31:47PM -0400, Jerome Glisse wrote: > > > Not doable as page refcount can change for things unrelated to GUP, with > > > John changes we can identify GUP and we could potentialy copy GUPed page > > > instead of COW but this can potentialy slow down fork() and i am not sure > > > how acceptable this would be. Also this does not solve GUP against page > > > that are already in fork tree ie page P0 is in process A which forks, > > > we now have page P0 in process A and B. Now we have process A which forks > > > again and we have page P0 in A, B, and C. Here B and C are two branches > > > with root in A. B and/or C can keep forking and grow the fork tree. > > > > For a long time now RDMA has broken COW pages when creating user DMA > > regions. > > > > The problem has been that fork re-COW's regions that had their COW > > broken. > > > > So, if you break the COW upon mapping and prevent fork (and others) > > from copying DMA pinned then you'd cover the cases. > > I am not sure we want to prevent COW for pinned GUP pages, this would > change current semantic and potentialy break/slow down existing apps. Isn't that basically exactly what 17839856fd588 does? It looks like it uses the same approach RDMA does by sticking FOLL_WRITE even though it is a read action. After that change the reamining bug is that fork can re-establish a COW./ > Anyway i think we focus too much on fork/COW, it is just an unfixable > broken corner cases, mmu notifier allows you to avoid it. Forcing real > copy on fork would likely be seen as regression by most people. If you don't copy the there are data corruption bugs though. Real apps probably don't hit a problem here as they are not forking while GUP's are active (RDMA excluded, which does do this) I think that implementing page pinning by blocking mmu notifiers for the duration of the pin is a particularly good idea either, that actually seems a lot worse than just having the pin in the first place. Particularly if it is only being done to avoid corner case bugs that already afflict other GUP cases :( > > What do you mean 'GUP fast is still succeptible to this' ? > > Not all GUP fast path are updated (intentionaly) __get_user_pages_fast() Sure, that is is the 'raw' accessor Jason _______________________________________________ amd-gfx mailing list amd-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/amd-gfx