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Subject: Re: [PATCH] mm, thp: relax migration wait when failed to get tail
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To:     Matthew Wilcox <willy@infradead.org>
Cc:     Hugh Dickins <hughd@google.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>
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>
 <71c320bf-3fcb-f8c0-65e4-ff706af05607@linux.alibaba.com>
 <YLdyVEbPs+Gwf3G4@casper.infradead.org>
From:   Yu Xu <xuyu@linux.alibaba.com>
Message-ID: <1c6b8331-3b74-83fd-abdb-4d7d8f19eb9d@linux.alibaba.com>
Date:   Wed, 2 Jun 2021 20:59:33 +0800
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On 6/2/21 7:58 PM, Matthew Wilcox wrote:
> 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.

Thanks. I have recalled that head page is frozen when splitting THP.

-- 
Thanks,
Yu