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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Date:	Mon, 23 Jan 2012 16:46:21 +0000
From:	Niels de Vos <ndevos@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Bryn M. Reeves" <bmr@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [PATCH v2] fs: Invalidate the cache for a parent block-device
 if fsync() is called for a partition

On 01/23/2012 04:27 PM, Jeff Moyer wrote:
> Niels de Vos <ndevos@...hat.com> writes:
> 
>> 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.
> 
> What problem, exactly, are you trying to fix?  Could you please post a
> reproducer?

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.

I hope this explains is clear enough, if not, please let me know.

Thanks,
Niels
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