Matthew Wilcox willy@infradead.org wrote:
On Tue, Sep 24, 2024 at 09:28:44AM +0900, Jeongjun Park wrote:
Matthew Wilcox willy@infradead.org 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