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Date:   Tue, 1 Jun 2021 21:27:12 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Xu Yu <xuyu@linux.alibaba.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        gavin.dg@linux.alibaba.com, Greg Thelen <gthelen@google.com>,
        Wei Xu <weixugc@google.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm, thp: relax migration wait when failed to get tail
 page
Message-ID: <YLaYIAeNUlitUy0z@casper.infradead.org>
References: <bc8567d7a2c08ab6fdbb8e94008157265d5d28a3.1622564942.git.xuyu@linux.alibaba.com>
 <alpine.LSU.2.11.2106010947370.1090@eggly.anvils>
 <YLZqKJ4anEGpAZfp@casper.infradead.org>
 <alpine.LSU.2.11.2106011114580.1045@eggly.anvils>
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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.