linux-kernel - Re: [PATCH] mm: fix possible cause of a page

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Message-ID: <BANLkTim-hyXpLj537asC__8exMo3o-WCLA@mail.gmail.com>
Date:	Thu, 7 Apr 2011 14:41:53 +0200
From:	Robert Święcki <robert@...ecki.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Michel Lespinasse <walken@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG

>>> I was about to send you my own UNTESTED patch: let me append it anyway,
>>> I think it is more correct than yours (it's the offset of vm_end we need
>>> to worry about, and there's the funny old_len,new_len stuff).
>>
>> Umm. That's what my patch did too. The
>>
>>   pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
>>
>> is the "offset of the pgoff" from the original mapping, then we do
>>
>>   pgoff += vma->vm_pgoff;
>>
>> to get the pgoff of the new mapping, and then we do
>>
>>   if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
>>
>> to check that the new mapping is ok.
>>
>> I think yours is equivalent, just a different (and odd - that
>> linear_page_index() thing will do lots of unnecessary shifts and
>> hugepage crap) way of writing it.
>>
>>> See what you think - sorry, I'm going out now.
>>
>> I think _yours_ is conceptually buggy, because I think that test for
>> "vma->vm_file" is wrong.
>>
>> Yes, new anonymous mappings set vm_pgoff to the virtual address, but
>> that's not true for mremap() moving them around, afaik.
>>
>> Admittedly it's really hard to get to the overflow case, because the
>> address is shifted down, so even if you start out with an anonymous
>> mmap at a high address (to get a big vm_off), and then move it down
>> and expand it (to get a big size), I doubt you can possibly overflow.
>> But I still don't think that the test for vm_file is semantically
>> sensible, even if it might not _matter_.
>>
>> But whatever. I suspect both our patches are practically doing the
>> same thing, and it would be interesting to hear if it actually fixes
>> the issue. Maybe there is some other way to mess up vm_pgoff that I
>> can't think of right now.
>
> Testing with Linus' patch. Will let you know in a few hours.

Ok, nothing happened after ~20h. The bug, usually, was triggered within 5-10h.

I can add some printk in this condition, and let it run for a few days
(I will not have access to my testing machine throughout that time),
if you think this will confirm your hypothesis.

-- 
Robert Święcki
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