block: Invalidate the cache for a parent block-device
 

Hi

This definitely doesn't belong to blkdev_issue_flush.

blkdev_issue_flush is called by filesystems each time they want to
synchronize hardware disk cache (for example, when committing a journal).

The patch may cause serious performance regressions (each time a
filesystem commits journal, the patch causes it to walk all buffers in the
whole-disk device).

You should put this code into fsync_bdev (so that it is called only on
fsync or BLKFLSBUF) and not to blkdev_issue_flush.

Mikulas

> Executing a BLKFLSBUF-ioctl on a partition flushes the caches for that
> partition but reading data through the parent device will still return
> the old cached data.
>
> The cache for the block-device is not synced if the block-device is kept
> open (due to a mounted partition, for example). Only when all users for
> the disk have exited, the cache for the disk is made consistent again.
>
> Calling invalidate_bdev() on the parent block-device in case
> blkdev_issue_flush() was called for a partition fixes this.
>
> The problem can be worked around by forcing the caches to be flushed
> with either
> # blockdev --flushbufs ${dev_disk}
> or
> # echo 3 > /proc/sys/vm/drop_caches
>
> CC: Bryn M. Reeves <bmr <at> redhat.com>
> Signed-off-by: Niels de Vos <ndevos <at> redhat.com>
> ---
> block/blk-flush.c | 3 +++
> 1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 720ad60..e876f8e 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -448,6 +448,9 @@ int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_
>
> if (!bio_flagged(bio, BIO_UPTODATE))
> ret = -EIO;
> + else if (bdev != bdev->bd_contains)
> + /* invalidate parent block_device */
> + invalidate_bdev(bdev->bd_contains);
>
> bio_put(bio);
> return ret;
> --
> 1.7.6.5
>

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