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    SubjectRe: [PATCH 1/7] block: Add block_flush_device()
    Bartlomiej Zolnierkiewicz wrote:
    > On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
    >> This patch adds a helper function that should be used by filesystems that need
    >> to flush the underlying block device on fsync()/fdatasync().
    >>
    >> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
    >> ---
    >>
    >> diff -urNp linux-2.6.29-orig/fs/buffer.c linux-2.6.29/fs/buffer.c
    >> --- linux-2.6.29-orig/fs/buffer.c 2009-03-24 08:12:14.000000000 +0900
    >> +++ linux-2.6.29/fs/buffer.c 2009-03-30 15:27:04.000000000 +0900
    >> @@ -165,6 +165,17 @@ void end_buffer_write_sync(struct buffer
    >> put_bh(bh);
    >> }
    >>
    >> +/* Issue flush of write caches on the block device */
    >> +int block_flush_device(struct block_device *bdev)
    >
    > I don't consider this an improvement over using blkdev_issue_flush().

    The reason I used a wrapper is that I did not like the semantics provided
    by blkdev_issue_flush(). On the one hand, I did not want to pass -EOPNOTSUPP
    to filesystems (it is not an error filesystems should care about). On the
    other hand it is weird that some filesystems use blkdev_issue_flush() when
    they want emit a barrier. blkdev_issue_flush() happens to be implemented
    as an empty (block layer) barrier, but I think that is an implementation
    detail filesystems should not neet to know about. Indeed I am working on a
    patch that implements blkdev_issue_empty_barrier(), so that we can optimize
    fsync() flushes and filesystem-originated barriers independently in the block
    layer.

    Judging from your comments below, it seems we are in the same page regarding
    this issue.

    Again, thank you for you feedback!

    - Fernando

    >> +{
    >> + int ret = 0;
    >> +
    >> + ret = blkdev_issue_flush(bdev, NULL);
    >
    > The problem lies in using NULL for error_sector argument which shows
    > a subtle deficiency of the current implementation/usage of barriers
    > based on a write cache flushing.
    >
    > I intend to document the issue with adding the FIXME to the current
    > users of blkdev_issue_flush() so the problem is at least known and not
    > forgotten (fixing it would require some work from both block and fs
    > sides and unfortunately there wasn't even a willingness to discuss
    > possible solutions few years back when the original code was added).
    >
    > Thanks,
    > Bart

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