Re: [PATCH] mm: migrate: fix data-race in migrate_folio_unmap()
From: Jeongjun Park
Date: Tue Sep 24 2024 - 00:50:17 EST
> 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