On Tue, Jun 01, 2021 at 12:10:48PM -0700, Hugh Dickins wrote: > On Tue, 1 Jun 2021, Matthew Wilcox wrote: > > On Tue, Jun 01, 2021 at 09:55:56AM -0700, Hugh Dickins wrote: > > > > > > Well caught: you're absolutely right that there's a bug there. > > > But isn't cond_resched() just papering over the real bug, and > > > what it should do is a "page = compound_head(page);" before the > > > get_page_unless_zero()? How does that work out in your testing? > > > > You do realise you're strengthening my case for folios by suggesting > > that, don't you? ;-) > > Hah! Well, I do realize that I'm offering you a marketing opportunity. > And you won't believe how many patches I dread to post for fear of that ;-) > > But I'm not so sure that it strengthens your case: apparently folios > had not detected this? Or do you have a hoard of folio-detected fixes > waiting for the day, and a folio-kit for each of the stable releases? Oh, I wish! I haven't been working on converting the migration code to use folios. If I'd taken the step to convert put_and_wait_on_page_locked() to folio_put_and_wait_locked(), I would have fixed the bug, but I'm not sure I would have noticed that it was fixing a bug. I would have probably converted migration_entry_to_page() to be migration_entry_to_folio() and just inadvertently fixed it. > > I was going to suggest that it won't make any difference because the > > page reference count is frozen, but the freezing happens after the call > > to unmap_page(), so it may make a difference. > > I think that's a good point: I may have just jumped on the missing > compound_head(), without thinking it through as far as you have. > > I'm having trouble remembering the dynamics now; but I think there > are cond_resched()s in the unmap_page() part, so the splitter may > get preempted even on a non-preempt kernel; whereas the frozen > part is all done expeditiously, with interrupts disabled. That would certainly make a difference.
