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
