Messages in this thread | | | Subject | Re: [PATCH] mm, thp: relax migration wait when failed to get tail page | From | Yu Xu <> | Date | Wed, 2 Jun 2021 11:27:47 +0800 |
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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
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