On Fri, 23 Jul 2021 at 19:40, Christian König ckoenig.leichtzumerken@gmail.com wrote:
Am 23.07.21 um 11:21 schrieb Daniel Vetter:
On Fri, Jul 23, 2021 at 11:13 AM Christian König ckoenig.leichtzumerken@gmail.com wrote:
Am 23.07.21 um 10:47 schrieb Daniel Vetter:
On Thu, Jul 22, 2021 at 02:41:23PM +0200, Christian König wrote:
Doing this in vmw_ttm_destroy() is to late.
It turned out that this is not a good idea at all because it leaves pointers to freed up system memory pages in the GART tables of the drivers.
So I wanted to review this series, and I can't reconcile your claim here with the demidlayering Dave has done. The driver patches here don't ouright undo what Dave has done, but that means the bug has been preexisting since forever (or is due to some other change?), and your commit message is a bit confusing here.
The final patch just undoes the demidlayering from Dave, and I really don't see where there's even a functional change there.
And even these patches here don't really change a hole lot with the calling sequence for at least final teardown: ttm_tt_destroy_common calls ttm_tt_unpopulate as the first thing, so at least there there's no change.
Can you pls elaborate more clearly what exactly you're fixing and what exactly needs to be reordered and where this bug is from (commit sha1)? As is I'm playing detective and the evidence presented is extremely since and I can't reconcile it at all.
I mean I know you don't like typing commit message and documentation, but it does get occasionally rather frustrating on the reviewer side if I have to interpolate between some very sparse hints for this stuff :-/
Yeah, when have seen the history it's rather obvious what's wrong here and I expected Dave to review it himself.
Previously we had three states in TTM for a tt object: Allocated -> Populated -> Bound which on destruction where done in the order unbind -> unpopulate -> free.
Dave moved handling of the bound state into the drivers since it is basically a driver decision and not a TTM decision what should be bound and what not (that part perfectly makes sense).
I haven't reviewed all the patches from Dave, only the one you pointed at (in the last patch). And that one I still can't match up with your description. If there's other commits relevant, can you pls dig them out?
Like it all makes sense what you're saying and matches the code, I just can't match it up with the commit you're referencing.
That is the patch directly following the one I've mentioned:
commit 37bff6542c4e140a11657406c1bab50a40329cc1 Author: Dave Airlie airlied@redhat.com Date: Thu Sep 17 13:24:50 2020 +1000
drm/ttm: move unbind into the tt destroy. This moves unbind into the driver side on destroy paths.I will add a Fixes tag to make that clear.
But this patch also just moves the undbind from the TTM destroy path to the driver destroy path.
To be honest I'm not 100% sure either when the when the unbind moved from the unpopulate path into the destroy path, but I think that this wasn't always the case. Let me try to dig that up.
The problem is that he also moved doing the unbind into the free callback instead of the unpopulate callback. This result in stale page pointers in the GART if that unpopulate operation isn't immediately followed by a free.
Thinking more about it if we do populated->unpopulated->populated then we would also have stale pointers to the old pages which is even worse.
This is also not de-midlayering since we already have a proper ttm_tt_init()/ttm_tt_fini() functions which should work nicely for the tt object.
Well you're last patch moves the ttm_tt_destroy_common stuff back into ttm, which kinda is de-demidlayering. So I'm confused.
Ah, yes that is correct. I've also considered to move this in ttm_tt_fini instead of there.
But that would be a larger change and I wanted to fix the problem at hand first, potentially even adding a CC stable tag.
Other bit: I think it'd be good to document this properly in the callbacks, and maybe ideally go about and kerneldoc-ify the entire ttm_tt.h header. Otherwise when we eventually (never?) get around to that, everyone has forgotten these semantic details and issues again.
Already working towards including more of the TTM headers and code files in kerneldoc. But not quite there yet.
But you know, normal human: Only equipped with one head and two hands and not cloneable.
I'm the same, but I'm not seeing where this problem happens at all, do we have any backtraces or demos for this?
I split bind/unbind into the driver, but the driver should still always be moving things to unbound states before an unpopulate is called, is there a driver doing something unexpected here?
at worst I'd like to see a WARN_ON put in first and a test in igt that triggers it, because right now I'm not see that path through the drivers/ttm that leads to unpopulated pages ending up happening while bound.
From 5.14-rc3 unpopulate is called from ttm_bo_pipeline_gutting in
non-ghost path and there is no unbind, pipeline gutting is called from evict/validate, when there is no placement suggested for the object, is this case getting hit somewhere without the driver having previously unbound things?
ttm_tt_destroy_common: calls unpopulate, everyone calls this directly after unbinding ttm_tt_swapout: calls unpopulate, we don't swapout objects from TT directly, we should always unbind first, this used to have an assert against that, ttm_tt_populate: call unpopulate in failure path
So any place I can see unpopulate getting called with a bound TT should be a bug, and fixed, we could protect against it better but I'm not seeing the need for this series to outright revert things back as helping.
Dave.