Re: [PATCH v1 4/4] mm/memory: document restore_exclusive_pte()

[Simona Vetter <simona.vetter@ffwll.ch>]

On Thu, Jan 30, 2025 at 10:37:06AM +0100, David Hildenbrand wrote:
> On 30.01.25 01:27, Alistair Popple wrote:
> > On Wed, Jan 29, 2025 at 12:58:02PM +0100, David Hildenbrand wrote:
> > > Let's document how this function is to be used, and why the requirement
> > > for the folio lock might maybe be dropped in the future.
> > 
> > Sorry, only just catching up on your other thread. The folio lock was to ensure
> > the GPU got a chance to make forward progress by mapping the page. Without it
> > the CPU could immediately invalidate the entry before the GPU had a chance to
> > retry the fault.
> > > Obviously performance wise having such thrashing is terrible, so should
> > really be avoided by userspace, but the lock at least allowed such programs
> > to complete.
> 
> Thanks for the clarification. So it's relevant that the MMU notifier in
> remove_device_exclusive_entry() is sent after taking the folio lock.
> 
> However, as soon as we drop the folio lock, remove_device_exclusive_entry()
> will become active, lock the folio and trigger the MMU notifier.
> 
> So the time it is actually mapped into the device is rather

Looks like you cut off a bit here (or mail transport did that somewhere),
but see my other reply I don't think this is a legit use-case. So we don't
have to worry. Well beyond documenting that if userspace concurrently thrashes
the same page with both device atomics and cpu access it will stall real
bad.
-Sima
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch