> Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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. > Thanks for the explanation. Then it seems appropriate to annotate data-race in __folio_test_movable() so that KCSAN ignores it. I will apply the change and send you a new patch. Regards, Jeongjun Park
