On Tue, Sep 24, 2024 at 09:28:44AM +0900, Jeongjun Park wrote: > > Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Mon, Sep 23, 2024 at 05:56:40PM +0200, David Hildenbrand wrote: > >>> On 22.09.24 17:17, Jeongjun Park wrote: > >>> I found a report from syzbot [1] > >>> > >>> When __folio_test_movable() is called in migrate_folio_unmap() to read > >>> folio->mapping, a data race occurs because the folio is read without > >>> protecting it with folio_lock. > >>> > >>> This can cause unintended behavior because folio->mapping is initialized > >>> to a NULL value. Therefore, I think it is appropriate to call > >>> __folio_test_movable() under the protection of folio_lock to prevent > >>> data-race. > >> > >> We hold a folio reference, would we really see PAGE_MAPPING_MOVABLE flip? > >> Hmm > > > > No; this shows a page cache folio getting truncated. It's fine; really > > a false alarm from the tool. I don't think the proposed patch > > introduces any problems, but it's all a bit meh. > > > > Well, I still don't understand why it's okay to read folio->mapping > without folio_lock . Because it can't be changed in a way which changes the value of __folio_test_movable(). We have a refcount on the folio at this point, so it can't be freed. And __folio_set_movable() happens at allocation.
