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

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

Quoting Tvrtko Ursulin (2019-07-17 14:09:00)
> 
> On 16/07/2019 16:37, Chris Wilson wrote:
> > 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???
> 
> Would page->mapping work? What is it telling us?

It basically tells us if there is a fs around; anything that is the most
basic of malloc (even tmpfs/shmemfs has 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().
> 
> Is this a problem? On a path with mappings we trylock and so solve the 
> set_dirty_locked and recursive deadlock issues, and with no mappings 
> with always dirty the page and avoid data corruption.

The problem as I see it is !page->mapping are likely an insignificant
minority of userptr; as I think even memfd are essentially shmemfs (or
hugetlbfs) and so have mappings.
-Chris