Re: [PATCH] mm: non-atomically mark page accessed during page cache allocation where possible -fix

[Mel Gorman]

On Tue, May 20, 2014 at 12:34:53PM -0700, Andrew Morton wrote:
> On Tue, 20 May 2014 16:49:00 +0100 Mel Gorman <mgorman@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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