Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page()

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2019-07-16 16:25:22)
> 
> On 16/07/2019 13:49, Chris Wilson wrote:
> > Following a try_to_unmap() we may want to remove the userptr and so call
> > put_pages(). However, try_to_unmap() acquires the page lock and so we
> > must avoid recursively locking the pages ourselves -- which means that
> > we cannot safely acquire the lock around set_page_dirty(). Since we
> > can't be sure of the lock, we have to risk skip dirtying the page, or
> > else risk calling set_page_dirty() without a lock and so risk fs
> > corruption.
> 
> So if trylock randomly fail we get data corruption in whatever data set 
> application is working on, which is what the original patch was trying 
> to avoid? Are we able to detect the backing store type so at least we 
> don't risk skipping set_page_dirty with anonymous/shmemfs?

page->mapping???

We still have the issue that if there is a mapping we should be taking
the lock, and we may have both a mapping and be inside try_to_unmap().

I think it's lose-lose. The only way to win is not to userptr :)
-Chris