On Mon, 23 Jan 2012, Niels de Vos wrote:
> Executing an fsync() on a file-descriptor of a partition flushes the
> caches for that partition by calling blkdev_issue_flush(). However, it
> seems that 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_fsync() 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@xxxxxxxxxx>
> CC: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Signed-off-by: Niels de Vos <ndevos@xxxxxxxxxx>
>
Acked-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> ---
> v2:
> - Do not call invalidate_bdev() from blkdev_issue_flush() and prevent
> performance degration with journalled filesystems.
>
> Suggested was to call invalidate_bdev() in fsync_bdev(), but this is
> not in the call-path of mkfs.ext3 and similar tools. Hence the issue
> persists.
>
> - Correct phrasing a little, changing ioctl-BLKFLSBUF is not required.
>
> - This issue also occurs when doing an ioctl-BLKFLSBUF on a partition.
> Reading the whole disk will still return cached data. If this is an
> issue, it will need a seperate patch.
> ---
> fs/block_dev.c | 4 ++++
> 1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 0e575d1..433c4de 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -424,6 +424,10 @@ int blkdev_fsync(struct file *filp, loff_t start, loff_t end, int datasync)
> if (error == -EOPNOTSUPP)
> error = 0;
>
> + /* invalidate parent block_device */
> + if (!error && bdev != bdev->bd_contains)
> + invalidate_bdev(bdev->bd_contains);
> +
> return error;
> }
> EXPORT_SYMBOL(blkdev_fsync);
> --
> 1.7.6.5
>
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