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Date:	Thu, 22 May 2014 11:05:39 +0530
From:	Prabhakar Lad <prabhakar.csengg@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] mm: non-atomically mark page accessed during page cache
 allocation where possible -fix

On Wed, May 21, 2014 at 1:04 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Tue, 20 May 2014 16:49:00 +0100 Mel Gorman <mgorman@...e.de> wrote:
>
>> Prabhakar Lad reported the following problem
>>
>>   I see following issue on DA850 evm,
>>   git bisect points me to
>>   commit id: 975c3a671f11279441006a29a19f55ccc15fb320
>>   ( mm: non-atomically mark page accessed during page cache allocation
>>   where possible)
>>
>>   Unable to handle kernel paging request at virtual address 30e03501
>>   pgd = c68cc000
>>   [30e03501] *pgd=00000000
>>   Internal error: Oops: 1 [#1] PREEMPT ARM
>>   Modules linked in:
>>   CPU: 0 PID: 1015 Comm: network.sh Not tainted 3.15.0-rc5-00323-g975c3a6 #9
>>   task: c70c4e00 ti: c73d0000 task.ti: c73d0000
>>   PC is at init_page_accessed+0xc/0x24
>>   LR is at shmem_write_begin+0x54/0x60
>>   pc : [<c0088aa0>]    lr : [<c00923e8>]    psr: 20000013
>>   sp : c73d1d90  ip : c73d1da0  fp : c73d1d9c
>>   r10: c73d1dec  r9 : 00000000  r8 : 00000000
>>   r7 : c73d1e6c  r6 : c694d7bc  r5 : ffffffe4  r4 : c73d1dec
>>   r3 : c73d0000  r2 : 00000001  r1 : 00000000  r0 : 30e03501
>>   Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
>>   Control: 0005317f  Table: c68cc000  DAC: 00000015
>>   Process network.sh (pid: 1015, stack limit = 0xc73d01c0)
>>
>> pagep is set but not pointing to anywhere valid as it's an uninitialised
>> stack variable. This patch is a fix to
>> mm-non-atomically-mark-page-accessed-during-page-cache-allocation-where-possible.patch
>>
>> ...
>>
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -2459,7 +2459,7 @@ ssize_t generic_perform_write(struct file *file,
>>               flags |= AOP_FLAG_UNINTERRUPTIBLE;
>>
>>       do {
>> -             struct page *page;
>> +             struct page *page = NULL;
>>               unsigned long offset;   /* Offset into pagecache page */
>>               unsigned long bytes;    /* Bytes to write to page */
>>               size_t copied;          /* Bytes copied from user */
>
> Well not really.  generic_perform_write() only touches *page if
> ->write_begin() returned "success", which is reasonable behavior.
>
> I'd say you mucked up shmem_write_begin() - it runs
> init_page_accessed() even if shmem_getpage() returned an error.  It
> shouldn't be doing that.
>
> This?
>
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: mm/shmem.c: don't run init_page_accessed() against an uninitialised pointer
>
> If shmem_getpage() returned an error then it didn't necessarily initialise
> *pagep.  So shmem_write_begin() shouldn't be playing with *pagep in this
> situation.
>
> Fixes an oops when "mm: non-atomically mark page accessed during page
> cache allocation where possible" (quite reasonably) left *pagep
> uninitialized.
>
> Reported-by: Prabhakar Lad <prabhakar.csengg@...il.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>
>  mm/shmem.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff -puN mm/shmem.c~mm-non-atomically-mark-page-accessed-during-page-cache-allocation-where-possiblefix-2 mm/shmem.c
> --- a/mm/shmem.c~mm-non-atomically-mark-page-accessed-during-page-cache-allocation-where-possiblefix-2
> +++ a/mm/shmem.c
> @@ -1376,7 +1376,7 @@ shmem_write_begin(struct file *file, str
>         struct inode *inode = mapping->host;
>         pgoff_t index = pos >> PAGE_CACHE_SHIFT;
>         ret = shmem_getpage(inode, index, pagep, SGP_WRITE, NULL);
> -       if (*pagep)
> +       if (ret == 0 && *pagep)
>                 init_page_accessed(*pagep);
>         return ret;
>  }

Reported-and-Tested-by: Lad, Prabhakar <prabhakar.csengg@...il.com>

Regards,
--Prabhakar Lad

> _
>
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