Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jun 19, 2020 at 10:10 PM Jerome Glisse <jglisse@redhat.com> wrote:
>
> On Fri, Jun 19, 2020 at 03:18:49PM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 19, 2020 at 02:09:35PM -0400, Jerome Glisse wrote:
> > > On Fri, Jun 19, 2020 at 02:23:08PM -0300, Jason Gunthorpe wrote:
> > > > On Fri, Jun 19, 2020 at 06:19:41PM +0200, Daniel Vetter wrote:
> > > >
> > > > > The madness is only that device B's mmu notifier might need to wait
> > > > > for fence_B so that the dma operation finishes. Which in turn has to
> > > > > wait for device A to finish first.
> > > >
> > > > So, it sound, fundamentally you've got this graph of operations across
> > > > an unknown set of drivers and the kernel cannot insert itself in
> > > > dma_fence hand offs to re-validate any of the buffers involved?
> > > > Buffers which by definition cannot be touched by the hardware yet.
> > > >
> > > > That really is a pretty horrible place to end up..
> > > >
> > > > Pinning really is right answer for this kind of work flow. I think
> > > > converting pinning to notifers should not be done unless notifier
> > > > invalidation is relatively bounded.
> > > >
> > > > I know people like notifiers because they give a bit nicer performance
> > > > in some happy cases, but this cripples all the bad cases..
> > > >
> > > > If pinning doesn't work for some reason maybe we should address that?
> > >
> > > Note that the dma fence is only true for user ptr buffer which predate
> > > any HMM work and thus were using mmu notifier already. You need the
> > > mmu notifier there because of fork and other corner cases.
> >
> > I wonder if we should try to fix the fork case more directly - RDMA
> > has this same problem and added MADV_DONTFORK a long time ago as a
> > hacky way to deal with it.
> >
> > Some crazy page pin that resolved COW in a way that always kept the
> > physical memory with the mm that initiated the pin?
>
> Just no way to deal with it easily, i thought about forcing the
> anon_vma (page->mapping for anonymous page) to the anon_vma that
> belongs to the vma against which the GUP was done but it would
> break things if page is already in other branch of a fork tree.
> Also this forbid fast GUP.
>
> Quite frankly the fork was not the main motivating factor. GPU
> can pin potentialy GBytes of memory thus we wanted to be able
> to release it but since Michal changes to reclaim code this is
> no longer effective.

What where how? My patch to annote reclaim paths with mmu notifier
possibility just landed in -mm, so if direct reclaim can't reclaim mmu
notifier'ed stuff anymore we need to know.

Also this would resolve the entire pain we're discussing in this
thread about dma_fence_wait deadlocking against anything that's not
GFP_ATOMIC ...
-Daniel

>
> User buffer should never end up in those weird corner case, iirc
> the first usage was for xorg exa texture upload, then generalize
> to texture upload in mesa and latter on to more upload cases
> (vertices, ...). At least this is what i remember today. So in
> those cases we do not expect fork, splice, mremap, mprotect, ...
>
> Maybe we can audit how user ptr buffer are use today and see if
> we can define a usage pattern that would allow to cut corner in
> kernel. For instance we could use mmu notifier just to block CPU
> pte update while we do GUP and thus never wait on dma fence.
>
> Then GPU driver just keep the GUP pin around until they are done
> with the page. They can also use the mmu notifier to keep a flag
> so that the driver know if it needs to redo a GUP ie:
>
> The notifier path:
>    GPU_mmu_notifier_start_callback(range)
>         gpu_lock_cpu_pagetable(range)
>         for_each_bo_in(bo, range) {
>             bo->need_gup = true;
>         }
>         gpu_unlock_cpu_pagetable(range)
>
>    GPU_validate_buffer_pages(bo)
>         if (!bo->need_gup)
>             return;
>         put_pages(bo->pages);
>         range = bo_vaddr_range(bo)
>         gpu_lock_cpu_pagetable(range)
>         GUP(bo->pages, range)
>         gpu_unlock_cpu_pagetable(range)
>
>
> Depending on how user_ptr are use today this could work.
>
>
> > (isn't this broken for O_DIRECT as well anyhow?)
>
> Yes it can in theory, if you have an application that does O_DIRECT
> and fork concurrently (ie O_DIRECT in one thread and fork in another).
> Note that O_DIRECT after fork is fine, it is an issue only if GUP_fast
> was able to lookup a page with write permission before fork had the
> chance to update it to read only for COW.
>
> But doing O_DIRECT (or anything that use GUP fast) in one thread and
> fork in another is inherently broken ie there is no way to fix it.
>
> See 17839856fd588f4ab6b789f482ed3ffd7c403e1f
>
> >
> > How does mmu_notifiers help the fork case anyhow? Block fork from
> > progressing?
>
> It enforce ordering between fork and GUP, if fork is first it blocks
> GUP and if forks is last then fork waits on GUP and then user buffer
> get invalidated.
>
> >
> > > I probably need to warn AMD folks again that using HMM means that you
> > > must be able to update the GPU page table asynchronously without
> > > fence wait.
> >
> > It is kind of unrelated to HMM, it just shouldn't be using mmu
> > notifiers to replace page pinning..
>
> Well my POV is that if you abide by rules HMM defined then you do
> not need to pin pages. The rule is asynchronous device page table
> update.
>
> Pinning pages is problematic it blocks many core mm features and
> it is just bad all around. Also it is inherently broken in front
> of fork/mremap/splice/...
>
> Cheers,
> Jérôme
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch