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
