Re: Known and unfixed active data loss bug in MM + XFS with large folios since Dec 2021 (any kernel from 6.1 upwards)

Davidlohr Bueso
2024-09-30T23:22:57Z

On Mon, 30 Sep 2024, Matthew Wilcox wrote:\n
>On Mon, Sep 30, 2024 at 01:12:37PM -0700, Linus Torvalds wrote:
>> It's basically been that way forever. The code has changed many times,
>> but we've basically always had that "wait on bit will wait not until
>> the next wakeup, but until it actually sees the bit being clear".
>>
>> And by "always" I mean "going back at least to before the git tree". I
>> didn't search further. It's not new.
>>
>> The only reason I pointed at that (relatively recent) commit from 2021
>> is that when we rewrote the page bit waiting logic (for some unrelated
>> horrendous scalability issues with tens of thousands of pages on wait
>> queues), the rewritten code _tried_ to not do it, and instead go "we
>> were woken up by a bit clear op, so now we've waited enough".
>>
>> And that then caused problems as explained in that commit c2407cf7d22d
>> ("mm: make wait_on_page_writeback() wait for multiple pending
>> writebacks") because the wakeups aren't atomic wrt the actual bit
>> setting/clearing/testing.
>
>Could we break out if folio->mapping has changed? Clearly if it has,
>we're no longer waiting for the folio we thought we were waiting for,
>but for a folio which now belongs to a different file.
>
>maybe this:
>
>+void __folio_wait_writeback(struct address_space *mapping, struct folio *folio)
>+{
>+ while (folio_test_writeback(folio) && folio->mapping == mapping) {

READ_ONCE(folio->mapping)?

>+ trace_folio_wait_writeback(folio, mapping);
>+ folio_wait_bit(folio, PG_writeback);
>+ }
>+}
>
>[...]
>
> void folio_wait_writeback(struct folio *folio)
> {
>- while (folio_test_writeback(folio)) {
>- trace_folio_wait_writeback(folio, folio_mapping(folio));
>- folio_wait_bit(folio, PG_writeback);
>- }
>+ __folio_wait_writeback(folio->mapping, folio);
> }

Also, the last sentence in the description would need to be dropped.

Thanks,
Davidlohr