From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jerome Glisse <jglisse@redhat.com>
Cc: "Felix Kuehling" <felix.kuehling@amd.com>,
linux-rdma <linux-rdma@vger.kernel.org>,
"Thomas Hellström (Intel)" <thomas_os@shipmail.org>,
"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
LKML <linux-kernel@vger.kernel.org>,
"DRI Development" <dri-devel@lists.freedesktop.org>,
"Christian König" <christian.koenig@amd.com>,
"moderated list:DMA BUFFER SHARING FRAMEWORK"
<linaro-mm-sig@lists.linaro.org>,
"Thomas Hellstrom" <thomas.hellstrom@intel.com>,
"amd-gfx list" <amd-gfx@lists.freedesktop.org>,
"Daniel Vetter" <daniel@ffwll.ch>,
"Daniel Vetter" <daniel.vetter@intel.com>,
"Mika Kuoppala" <mika.kuoppala@intel.com>,
"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
"open list:DMA BUFFER SHARING FRAMEWORK"
<linux-media@vger.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations
Date: Mon, 22 Jun 2020 21:02:31 -0300 [thread overview]
Message-ID: <20200623000231.GW6578@ziepe.ca> (raw)
In-Reply-To: <20200622201540.GB9708@redhat.com>
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
next prev parent reply other threads:[~2020-06-23 0:02 UTC|newest]
Thread overview: 101+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-04 8:12 [PATCH 00/18] dma-fence lockdep annotations, round 2 Daniel Vetter
2020-06-04 8:12 ` [PATCH 01/18] mm: Track mmu notifiers in fs_reclaim_acquire/release Daniel Vetter
2020-06-10 12:01 ` Thomas Hellström (Intel)
2020-06-10 12:25 ` [Intel-gfx] " Daniel Vetter
2020-06-10 19:41 ` [PATCH] " Daniel Vetter
2020-06-11 14:29 ` Jason Gunthorpe
2020-06-21 17:42 ` Qian Cai
2020-06-21 18:07 ` Daniel Vetter
2020-06-21 20:01 ` Daniel Vetter
2020-06-21 22:09 ` Qian Cai
2020-06-23 16:17 ` Qian Cai
2020-06-23 22:13 ` Daniel Vetter
2020-06-23 22:29 ` Qian Cai
2020-06-23 22:31 ` Dave Chinner
2020-06-23 22:36 ` Daniel Vetter
2020-06-21 17:00 ` [PATCH 01/18] " Qian Cai
2020-06-21 17:28 ` Daniel Vetter
2020-06-21 17:46 ` Qian Cai
2020-06-04 8:12 ` [PATCH 02/18] dma-buf: minor doc touch-ups Daniel Vetter
2020-06-10 13:07 ` Thomas Hellström (Intel)
2020-06-04 8:12 ` [PATCH 03/18] dma-fence: basic lockdep annotations Daniel Vetter
2020-06-04 8:57 ` Thomas Hellström (Intel)
2020-06-04 9:21 ` Daniel Vetter
[not found] ` <159126281827.25109.3992161193069793005@build.alporthouse.com>
2020-06-04 9:36 ` [Intel-gfx] " Daniel Vetter
2020-06-05 13:29 ` [PATCH] " Daniel Vetter
2020-06-05 14:30 ` Thomas Hellström (Intel)
2020-06-11 9:57 ` Maarten Lankhorst
2020-06-10 14:21 ` [Intel-gfx] [PATCH 03/18] " Tvrtko Ursulin
2020-06-10 15:17 ` Daniel Vetter
2020-06-11 10:36 ` Tvrtko Ursulin
2020-06-11 11:29 ` Daniel Vetter
2020-06-11 14:29 ` Tvrtko Ursulin
2020-06-11 15:03 ` Daniel Vetter
[not found] ` <159186243606.1506.4437341616828968890@build.alporthouse.com>
2020-06-11 8:44 ` Dave Airlie
2020-06-11 9:01 ` [Intel-gfx] " Daniel Stone
[not found] ` <159255511144.7737.12635440776531222029@build.alporthouse.com>
2020-06-19 8:51 ` Daniel Vetter
[not found] ` <159255801588.7737.4425728073225310839@build.alporthouse.com>
2020-06-19 9:43 ` Daniel Vetter
[not found] ` <159257233754.7737.17318605310513355800@build.alporthouse.com>
2020-06-22 9:16 ` Daniel Vetter
2020-07-09 7:29 ` Daniel Stone
2020-07-09 8:01 ` Daniel Vetter
2020-06-12 7:06 ` [PATCH] " Daniel Vetter
2020-06-04 8:12 ` [PATCH 04/18] dma-fence: prime " Daniel Vetter
2020-06-11 7:30 ` [Linaro-mm-sig] " Thomas Hellström (Intel)
2020-06-11 8:34 ` Daniel Vetter
2020-06-11 14:15 ` Jason Gunthorpe
2020-06-11 23:35 ` Felix Kuehling
2020-06-12 5:11 ` Daniel Vetter
2020-06-19 18:13 ` Jerome Glisse
2020-06-23 7:39 ` Daniel Vetter
2020-06-23 18:44 ` Felix Kuehling
2020-06-23 19:02 ` Daniel Vetter
2020-06-16 12:07 ` Daniel Vetter
2020-06-16 14:53 ` Jason Gunthorpe
2020-06-17 7:57 ` Daniel Vetter
2020-06-17 15:29 ` Jason Gunthorpe
2020-06-18 14:42 ` Daniel Vetter
2020-06-17 6:48 ` Daniel Vetter
2020-06-17 15:28 ` Jason Gunthorpe
2020-06-18 15:00 ` Daniel Vetter
2020-06-18 17:23 ` Jason Gunthorpe
2020-06-19 7:22 ` Daniel Vetter
2020-06-19 11:39 ` Jason Gunthorpe
2020-06-19 15:06 ` Daniel Vetter
2020-06-19 15:15 ` Jason Gunthorpe
2020-06-19 16:19 ` Daniel Vetter
2020-06-19 17:23 ` Jason Gunthorpe
2020-06-19 18:09 ` Jerome Glisse
2020-06-19 18:18 ` Jason Gunthorpe
2020-06-19 19:48 ` Felix Kuehling
2020-06-19 19:55 ` Jason Gunthorpe
2020-06-19 20:03 ` Felix Kuehling
2020-06-19 20:31 ` Jerome Glisse
2020-06-22 11:46 ` Jason Gunthorpe
2020-06-22 20:15 ` Jerome Glisse
2020-06-23 0:02 ` Jason Gunthorpe [this message]
2020-06-19 20:10 ` Jerome Glisse
2020-06-19 20:43 ` Daniel Vetter
2020-06-19 20:59 ` Jerome Glisse
2020-06-23 0:05 ` Jason Gunthorpe
2020-06-19 19:11 ` Alex Deucher
2020-06-19 19:30 ` Felix Kuehling
2020-06-19 19:40 ` Jerome Glisse
2020-06-19 19:51 ` Jason Gunthorpe
2020-06-12 7:01 ` [PATCH] " Daniel Vetter
2020-06-04 8:12 ` [PATCH 05/18] drm/vkms: Annotate vblank timer Daniel Vetter
2020-06-04 8:12 ` [PATCH 06/18] drm/vblank: Annotate with dma-fence signalling section Daniel Vetter
2020-06-04 8:12 ` [PATCH 07/18] drm/atomic-helper: Add dma-fence annotations Daniel Vetter
2020-06-04 8:12 ` [PATCH 08/18] drm/amdgpu: add dma-fence annotations to atomic commit path Daniel Vetter
2020-06-23 10:51 ` Daniel Vetter
2020-06-04 8:12 ` [PATCH 09/18] drm/scheduler: use dma-fence annotations in main thread Daniel Vetter
2020-06-04 8:12 ` [PATCH 10/18] drm/amdgpu: use dma-fence annotations in cs_submit() Daniel Vetter
2020-06-04 8:12 ` [PATCH 11/18] drm/amdgpu: s/GFP_KERNEL/GFP_ATOMIC in scheduler code Daniel Vetter
2020-06-04 8:12 ` [PATCH 12/18] drm/amdgpu: DC also loves to allocate stuff where it shouldn't Daniel Vetter
2020-06-04 8:12 ` [PATCH 13/18] drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail Daniel Vetter
2020-06-05 8:30 ` Pierre-Eric Pelloux-Prayer
2020-06-05 12:41 ` Daniel Vetter
2020-06-04 8:12 ` [PATCH 14/18] drm/scheduler: use dma-fence annotations in tdr work Daniel Vetter
2020-06-04 8:12 ` [PATCH 15/18] drm/amdgpu: use dma-fence annotations for gpu reset code Daniel Vetter
2020-06-04 8:12 ` [PATCH 16/18] Revert "drm/amdgpu: add fbdev suspend/resume on gpu reset" Daniel Vetter
2020-06-04 8:12 ` [PATCH 17/18] drm/amdgpu: gpu recovery does full modesets Daniel Vetter
2020-06-04 8:12 ` [PATCH 18/18] drm/i915: Annotate dma_fence_work Daniel Vetter
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