linux-kernel - Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

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Date:	Thu, 04 Jan 2007 16:07:18 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...l.org>
CC:	Andrea Gelmini <gelma@...ma.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

Linus Torvalds wrote:
> 
> On Thu, 4 Jan 2007, Nick Piggin wrote:
> 
>>Yhat's when the bug was introduced -- 2.6.19. 2.6.18 does not have
>>this bug, so it cannot be years old.
> 
> 
> Actually, I think 2.6.18 may have a subtle variation on it. 
> 
> In particular, I look back at the try_to_free_buffers() thing that I hated 
> so much, and it makes me wonder.. It used to do:
> 
> 	spin_lock(&mapping->private_lock);
> 	ret = drop_buffers(page, &buffers_to_free);
> 	spin_unlock(&mapping->private_lock);
> 	if (ret) {
> 		.. crappy comment ..
> 		if (test_clear_page_dirty(page))
> 			task_io_account_cancelled_write(PAGE_CACHE_SIZE);
> 	}
> 
> and I think that at least on SMP, we had a race with another CPU doing the 
> "mark page dirty if it was dirty in the PTE" at the same time. Because the 
> marking dirty would come in, find no buffers (they just got dropped), and 
> then mark the page dirty (ignoring the lack of any buffers), but then the 
> above would do the "test_clear_page_dirty()" thing on it.
> 
> Ie the race, I think, existed where that crappy comment was.
> 
> But that much older race would only trigger on SMP (or possibly UP with 
> preempt).

Oh yes the try_to_free_buffers race, I think, does exist in older kernels.
Yes according to our earlier analysis it would trigger with UP+preempt and
SMP.

But the patch that Andrea was pointing to was your last patch (The Fix),
which stopped page_mkclean caller throwing out dirty bits. You probably
didn't see that in the mail I cc'ed you on.

So yes it would be interesting to see whether fixing try_to_free_buffers
fixes Andrea's problem on older kernels.

-- 
SUSE Labs, Novell Inc.
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