linux-kernel - Re: PROBLEM: repeated remap_file_pages on tmpfs triggers bug on process exit

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Date:	Tue, 15 Jul 2014 15:33:29 +0400
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Ingo Korb <ingo.korb@...dortmund.de>,
	Ning Qu <quning@...gle.com>, Dave Jones <davej@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PROBLEM: repeated remap_file_pages on tmpfs triggers bug on
 process exit

On Tue, Jul 15, 2014 at 2:55 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> Konstantin Khlebnikov wrote:
>> It seems boundng logic in do_fault_around is wrong:
>>
>> start_addr = max(address & fault_around_mask(), vma->vm_start);
>> off = ((address - start_addr) >> PAGE_SHIFT) & (PTRS_PER_PTE - 1);
>> pte -= off;
>> pgoff -= off;
>>
>> Ok, off  <= 511, but it might be bigger than pte offset in pte table.
>
> I don't see how it possible: fault_around_mask() cannot be more than 0x1ff000
> (x86-64, fault_around_bytes == 2M). It means start_addr will be aligned to 2M
> boundary in this case which is start of the page table pte belong to.
>
> Do I miss something?

Nope, you're right. This fixes kernel crash but not the original problem.

Problem is caused by calling do_fault_around for _non-linear_ faiult.
In this case pgoff is shifted and might become negative during calculation.
I'll send another patch.

>
> --
>  Kirill A. Shutemov
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