Re: [PATCH 1/7] block: Add block_flush_device()

Fernando Luis Vázquez Cao
2009-03-31T06:10:18Z

Bartlomiej Zolnierkiewicz wrote:
> On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
>> This patch adds a helper function that should be used by filesystems that need
>> to flush the underlying block device on fsync()/fdatasync().
>>
>> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
>> ---
>>
>> diff -urNp linux-2.6.29-orig/fs/buffer.c linux-2.6.29/fs/buffer.c
>> --- linux-2.6.29-orig/fs/buffer.c 2009-03-24 08:12:14.000000000 +0900
>> +++ linux-2.6.29/fs/buffer.c 2009-03-30 15:27:04.000000000 +0900
>> @@ -165,6 +165,17 @@ void end_buffer_write_sync(struct buffer
>> put_bh(bh);
>> }
>>
>> +/* Issue flush of write caches on the block device */
>> +int block_flush_device(struct block_device *bdev)
>
> I don't consider this an improvement over using blkdev_issue_flush().

The reason I used a wrapper is that I did not like the semantics provided
by blkdev_issue_flush(). On the one hand, I did not want to pass -EOPNOTSUPP
to filesystems (it is not an error filesystems should care about). On the
other hand it is weird that some filesystems use blkdev_issue_flush() when
they want emit a barrier. blkdev_issue_flush() happens to be implemented
as an empty (block layer) barrier, but I think that is an implementation
detail filesystems should not neet to know about. Indeed I am working on a
patch that implements blkdev_issue_empty_barrier(), so that we can optimize
fsync() flushes and filesystem-originated barriers independently in the block
layer.

Judging from your comments below, it seems we are in the same page regarding
this issue.

Again, thank you for you feedback!

- Fernando

>> +{
>> + int ret = 0;
>> +
>> + ret = blkdev_issue_flush(bdev, NULL);
>
> The problem lies in using NULL for error_sector argument which shows
> a subtle deficiency of the current implementation/usage of barriers
> based on a write cache flushing.
>
> I intend to document the issue with adding the FIXME to the current
> users of blkdev_issue_flush() so the problem is at least known and not
> forgotten (fixing it would require some work from both block and fs
> sides and unfortunately there wasn't even a willingness to discuss
> possible solutions few years back when the original code was added).
>
> Thanks,
> Bart