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    SubjectRe: [PATCH] vfs: calculate block-size correctly
    From
    On Tue, Sep 14, 2010 at 7:09 AM, Andrew Morton
    <akpm@linux-foundation.org> wrote:
    >
    > On Wed, 8 Sep 2010 18:07:10 +0800
    > Cong Meng <mcpacino@gmail.com> wrote:
    >
    > > I sent this patch half mount ago, but no response at all. If anything is
    > > wrong, please tell me. Thanks a lots. This is my first time to send patch
    > > to linux kernel.
    > >
    > > The invocation of __getblk(bdev, block, size) will cause kernel stall if
    > > the @size parameter is not equal to the blockr-size of @bdev, which is saved
    > > in bdev->bd_inode->i_blkbits.
    > >
    > > submit_bh() has the similar problem.
    > >
    > > This patch calculates the block-size using i_blkbits.
    > > So that, for example, __getblk() can get a 4K buffer head while the
    > > block-size of @bdev is 1K.
    > >
    >
    > This comes up every couple of years and iirc we always decided that
    > it's a bug in the calling code somewhere.  How did you hit it?
    >

    I am working on a new multi-version snapshot target module for device-mapper
    which is almost done. It's on-disk metadata block size is fixed 4K, no matter
    what the actual disk block size is. So I call __getblk(bdev, block,
    4096) to buffer
    the metedata.

    This is how I hit it.

    grow_dev_page(), the main routine of grow_buffers(), has already supported to
    grow page buffer of different size.

    If we adapt grow_buffer() this way, imho, It's flexible that make
    __getblk() get
    buffer head of different size. why not?

    >
    > > --- a/fs/buffer.c
    > > +++ b/fs/buffer.c
    > > @@ -1051,10 +1051,7 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
    > >       pgoff_t index;
    > >       int sizebits;
    > >
    > > -     sizebits = -1;
    > > -     do {
    > > -             sizebits++;
    > > -     } while ((size << sizebits) < PAGE_SIZE);
    >
    > I'm sure the existing code could use __roundup_pow_of_two() here.
    >

    What does this mean?

    (sorry for sneding again. this is plain text.)

    Thanks.
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