Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

On Mon, Jan 6, 2014 at 11:47 AM, Motohiro Kosaki
<Motohiro.Kosaki@us.fujitsu.com> wrote:
>
>
>> -----Original Message-----
>> From: linus971@gmail.com [mailto:linus971@gmail.com] On Behalf Of Linus
>> Torvalds
>> Sent: Friday, January 03, 2014 7:18 PM
>> To: Vlastimil Babka
>> Cc: Sasha Levin; Andrew Morton; Wanpeng Li; Michel Lespinasse; Bob Liu;
>> Nick Piggin; Motohiro Kosaki JP; Rik van Riel; David Rientjes; Mel Gorman;
>> Minchan Kim; Hugh Dickins; Johannes Weiner; linux-mm; Linux Kernel Mailing
>> List
>> Subject: Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear
>> VMAs
>>
>> On Fri, Jan 3, 2014 at 3:36 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> >
>> > I'm for going with the removal of BUG_ON. The TestSetPageMlocked
>> > should provide enough race protection.
>>
>> Maybe. But dammit, that's subtle, and I don't think you're even right.
>>
>> It basically depends on mlock_vma_page() and munlock_vma_page() being
>> able to run CONCURRENTLY on the same page. In particular, you could have a
>> mlock_vma_page() set the bit on one CPU, and munlock_vma_page()
>> immediately clearing it on another, and then the rest of those functions
>> could run with a totally arbitrary interleaving when working with the exact
>> same page.
>>
>> They both do basically
>>
>>     if (!isolate_lru_page(page))
>>         putback_lru_page(page);
>>
>> but one or the other would randomly win the race (it's internally protected
>> by the lru lock), and *if* the munlock_vma_page() wins it, it would also do
>>
>>     try_to_munlock(page);
>>
>> but if mlock_vma_page() wins it, that wouldn't happen. That looks entirely
>> broken - you end up with the PageMlocked bit clear, but
>> try_to_munlock() was never called on that page, because
>> mlock_vma_page() got to the page isolation before the "subsequent"
>> munlock_vma_page().
>>
>> And this is very much what the page lock serialization would prevent.
>> So no, the PageMlocked in *no* way gives serialization. It's an atomic bit op,
>> yes, but that only "serializes" in one direction, not when you can have a mix
>> of bit setting and clearing.
>>
>> So quite frankly, I think you're wrong. The BUG_ON() is correct, or at least
>> enforces some kind of ordering. And try_to_unmap_cluster() is just broken
>> in calling that without the page being locked. That's my opinion. There may
>> be some *other* reason why it all happens to work, but no,
>> "TestSetPageMlocked should provide enough race protection" is simply not
>> true, and even if it were, it's way too subtle and odd to be a good rule.
>>
>> So I really object to just removing the BUG_ON(). Not with a *lot* more
>> explanation as to why these kinds of issues wouldn't matter.
>
> I don't have a perfect answer. But I can explain a bit history. Let's me try.
>
> First off, 5 years ago, Lee's original putback_lru_page() implementation required
> page-lock, but I removed the restriction months later. That's why we can see
> strange BUG_ON here.
>
> 5 years ago, both mlock(2) and munlock(2) called do_mlock() and it was protected  by
> mmap_sem (write mdoe). Then, mlock and munlock had no race.
> Now, __mm_populate() (called by mlock(2)) is only protected by mmap_sem read-mode. However it is enough to
> protect against munlock.
>
> Next, In case of mlock vs reclaim, the key is that mlock(2) has two step operation. 1) turn on VM_LOCKED under
> mmap_sem write-mode, 2) turn on Page_Mlocked under mmap_sem read-mode. If reclaim race against step (1),
> reclaim must lose because it uses trylock. On the other hand, if reclaim race against step (2), reclaim must detect
> VM_LOCKED because both VM_LOCKED modifier and observer take mmap-sem.
>
> By the way, page isolation is still necessary because we need to protect another page modification like page migration.
>
>
> My memory was alomostly flushed and I might lost some technical concern and past discussion. Please point me out,
> If I am overlooking something.

No. I did talk about completely different issue. My memory is
completely broken as I said. I need to read latest code and dig past
discussion. Sorry again, please ignore my last mail.

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