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

[Yu Xu]

On 6/2/21 3:10 AM, 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?
>
> > 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.
>
> Greg discovered the same issue recently, but we all got sidetracked,
> and I don't know where his investigation ended up.  He was in favour
> of cond_resched(), I was in favour of compound_head(); and I think I

I ever considered about using compound_head, but isn't there another
race that, the following put_and_wait_on_page_locked operates on the
"tail page" which has been split and is now a single page?

Anyway, I will test and verify compound_head.

> was coming to the conclusion that if cond_resched() is needed, it
> should not be there in __migration_entry_wait(), but somewhere up
> in mm/gup.c, so that other faults that retry, expecting to reschedule
> on return to userspace, do not get trapped in kernelspace this way.

Agreed. I will send v2, if cond_resched is still an option.

> Waiting on migration entries from THP splitting is an egregious
> example, but others may be suffering too.
>
> Hugh

--
Thanks,
Yu