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

[Matthew Wilcox]

On Wed, Jun 02, 2021 at 11:27:47AM +0800, Yu Xu wrote:
> 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?

No, having your own reference on a page prevents the page from being
split.  But that's a good question to ask.