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    SubjectRe: [PATCH] mm: non-atomically mark page accessed during page cache allocation where possible -fix
    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;
    }
    _


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