linux-kernel - Re: [PATCH v2] fs: Invalidate the cache for a parent block-device if fsync() is called for a partition

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Message-Id: <20120126020344.b3194916.akpm@linux-foundation.org>
Date:	Thu, 26 Jan 2012 02:03:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Niels de Vos <ndevos@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Bryn M. Reeves" <bmr@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [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@...hat.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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