Re: [GIT PULL] block/io bits for 2.6.35-rc

[Brian Bloniarz <bmb@athenacr.com>]

On 06/10/2010 12:25 PM, Jens Axboe wrote:
> On 2010-06-10 17:55, Linus Torvalds wrote:
>> On Thu, Jun 10, 2010 at 6:44 AM, Jens Axboe <jaxboe@fusionio.com> wrote:
>>>
>>> - A set of patches fixing the WB_SYNC_NONE writeback from Christoph. So
>>>  we should finally have both functional and working WB_SYNC_NONE from
>>>  umount context.
>>
>> I _really_ think this is too late, considering how broken it has been.
>> We already reverted the WB_SYNC_NONE things exactly because it didn't
>> work, didn't we? I'm going to be off-line in two days, and this part
>> of the pull request really makes me nervous, if only simply because of
>> the history of it all (ie it's always been broken, why shouldn't it be
>> broken now?).
>>
>> IOW, that's a lot of scary changes, that have historically not been
>> safe or sufficiently tested, and have caused problems for various
>> filesystems. Convince me why they should suddenly be ok to merge?
> 
> I agree, it's late and it makes me nervous too. I had them cook for
> a day, didn't see any problems. And Christoph would not send it in
> unless it passes at least xfs qa, which is what found the problems
> last time (the ones we reverted).
> 
> It's fixing a regression where umount takes a LONG time if you have
> a lot of dirty inodes, since it basically degenerates to a data
> integrity writeback instead of a simple WB_SYNC_NONE. If it wasn't
> fixing a nasty regression (the distros are all wanting a real fix
> for this, it's a user problem), I would not be submitting this code
> at this point in time.
> 

Reinforcing that last point: from what I could figure out, Fedora 13
is shipping the buggy WB_SYNC_NONE patch currently. Ubuntu 10.04 is
shipping an in-kernel workaround that has serious performance
drawbacks.

https://bugzilla.kernel.org/show_bug.cgi?id=15906 has links to the
downstream bugs.