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    SubjectRe: [PATCH] remove BUG() in possible but rare condition
    On Wed 11-04-12 15:10:24, Glauber Costa wrote:
    > While stressing the kernel with with failing allocations today,
    > I hit the following chain of events:
    >
    > alloc_page_buffers():
    >
    > bh = alloc_buffer_head(GFP_NOFS);
    > if (!bh)
    > goto no_grow; <= path taken
    >
    > grow_dev_page():
    > bh = alloc_page_buffers(page, size, 0);
    > if (!bh)
    > goto failed; <= taken, consequence of the above
    >
    > and then the failed path BUG()s the kernel.
    >
    > The failure is inserted a litte bit artificially, but even then,
    > I see no reason why it should be deemed impossible in a real box.
    >
    > Even though this is not a condition that we expect to see
    > around every time, failed allocations are expected to be handled,
    > and BUG() sounds just too much. As a matter of fact, grow_dev_page()
    > can return NULL just fine in other circumstances, so I propose we just
    > remove it, then.

    I am not familiar with the code much but a trivial call chain walk up to
    write_dev_supers (in btrfs) shows that we do not check for the return value
    from __getblk so we would nullptr and there might be more.
    I guess these need some treat before the BUG might be removed, right?

    >
    > Signed-off-by: Glauber Costa <glommer@parallels.com>
    > CC: Linus Torvalds <torvalds@linux-foundation.org>
    > CC: Andrew Morton <akpm@linux-foundation.org>
    > ---
    > fs/buffer.c | 1 -
    > 1 files changed, 0 insertions(+), 1 deletions(-)
    >
    > diff --git a/fs/buffer.c b/fs/buffer.c
    > index 36d6665..351e18e 100644
    > --- a/fs/buffer.c
    > +++ b/fs/buffer.c
    > @@ -985,7 +985,6 @@ grow_dev_page(struct block_device *bdev, sector_t block,
    > return page;
    >
    > failed:
    > - BUG();
    > unlock_page(page);
    > page_cache_release(page);
    > return NULL;
    > --
    > 1.7.7.6
    >
    > --
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    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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