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    SubjectRe: [PATCH] remove BUG() in possible but rare condition
    On Wed, 11 Apr 2012 15:10:24 -0300
    Glauber Costa <glommer@parallels.com> 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.
    >
    > 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;

    Cute.

    AFAICT what happened was that in my April 2002 rewrite of this code I
    put a non-fatal buffer_error() warning in that case to tell us that
    something bad happened.

    Years later we removed the temporary buffer_error() and mistakenly
    replaced that warning with a BUG(). Only it *can* happen.

    We can remove the BUG() and fix up callers, or we can pass retry=1 into
    alloc_page_buffers(), so grow_dev_page() "cannot fail". Immortal
    functions are a silly fiction, so we should remove the BUG() and fix up
    callers.



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