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SubjectRe: [PATCH v2] fs: Invalidate the cache for a parent block-device if fsync() is called for a partition
I suggest a viro cc on this one.

On Mon, 23 Jan 2012 10:38:29 +0000 Niels de Vos <ndevos@redhat.com> wrote:
> Executing an fsync() on a file-descriptor of a partition flushes the
> caches for that partition by calling blkdev_issue_flush(). However, it

The changelog is stale.

> 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

Please include your (useful) problem description in the changelog:

: The problem that was noticed is the following:
: 1) create two or more partitions on a device
: - use fdisk to create /dev/sdb1 and /dev/sdb2
: 2) format and mount one of the partition
: - mkfs -t ext3 /dev/sdb1
: 3) read through the main device to have something in the cache
: - read /dev/sdb with dd or use something like "parted /dev/sdb print"
: 4) now write something to /dev/sdb2, format the partition for example
: - mkfs -t ext3 /dev/sdb2
: 5) read the blocks where sdb2 starts, through /dev/sdb
: - use dd or do again a "parted /dev/sdb print"
:
: Without this patch, calling "blockdev --flushbufs" or dropping the
: caches, the result in 5) is the same as in 3). Reading the same area
: through /dev/sdb2 shows the inconsistancy between the two caches.
:
: With this patch, or one of the workarounds, the data read through
: /dev/sdb and /dev/sdb2 is the same.

>
> ...
>
> --- 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;
> }

It doesn't seem terribly logical to do this in blkdev_fsync(). Why not
do it right there in blkdev_ioctl()'s "case BLKFLSBUF"?

Bear in mind that invalidate_bdev() isn't a very strong function -
it won't drop pages which are dirty or under writeback nor pages which
others have a reference on. But I can see that this change would be a
best-effort user-convenience thing.



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