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    SubjectRe: [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@linux-foundation.org> wrote:
    > On Tue, 20 May 2014 16:49:00 +0100 Mel Gorman <mgorman@suse.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@linux-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@gmail.com>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > Cc: Vlastimil Babka <vbabka@suse.cz>
    > Cc: Jan Kara <jack@suse.cz>
    > Cc: Michal Hocko <mhocko@suse.cz>
    > Cc: Hugh Dickins <hughd@google.com>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Dave Hansen <dave.hansen@intel.com>
    > Cc: Mel Gorman <mgorman@suse.de>
    > Signed-off-by: Andrew Morton <akpm@linux-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@gmail.com>

    Regards,
    --Prabhakar Lad

    > _
    >


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