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

[Motohiro Kosaki]

> -----Original Message-----
> From: linus971@xxxxxxxxx [mailto:linus971@xxxxxxxxx] 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@xxxxxxx> 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.

Thanks.

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