Re: [PATCH] mm, thp: relax migration wait when failed to get tail page

[Matthew Wilcox]

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.