Re: [PATCH] ext4: fix ext4_evict_inode() racing against workqueue processing code

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Tue, Mar 26, 2013 at 09:34:03PM +0100, Jan Kara wrote:
> On Tue 26-03-13 13:52:51, Zheng Liu wrote:
> > Sorry for the late reply.
> > 
> > On Wed, Mar 20, 2013 at 10:45:23AM -0400, Theodore Ts'o wrote:
> > > On Wed, Mar 20, 2013 at 09:14:42AM -0500, Eric Sandeen wrote:
> > > > 
> > > > As an aside, is there any reason to have "dioread_nolock" as an option
> > > > at this point?  If it works now, would you ever *not* want it?
> > > > 
> > > > (granted it doesn't work with some journaling options etc, but that
> > > > behavior could be automatic, w/o the need for special mount options).
> > > 
> > > The primary restriction is that diread_nolock doesn't work when fs
> > > block size != page size.  If your proposal is that we automatically
> > > enable diread_nolock when we can use it safely, that's definitely
> > > something to consider for the next merge window.
> > 
> > Yes, I also think we can automatically enable dioread_nolock because it
> > brings us some benefits.
>   But isn't there also some overhead due to buffered writes having to go
> through uninit->init conversion?

Yeah, in my test, the IOPS will decrease after dioread_nolock enables.
But the latency of dio will also descrease.  Honestly I don't test
buffered IO.  So I will test this case and post the result later.  IMO,
this is a tradeoff that we want to improve latency or get a better
throughput.


> Plus there's this potential deadlock in
> dioread_nolock code (http://www.spinics.net/lists/linux-ext4/msg36569.html)
> which I'm not sure how to fix yet...

Yes, we need to fix this bug first.

> 
> > BTW, I think there is an minor improvement for dio overwrite codepath
> > with indirect-based file.  We don't need to take i_mutex in this
> > condition just as we have done for extent-based file.  If a user mounts
> > a ext2/3 file system with a ext4 kernel modules, he/she could get a
> > lower latency.  But it seems that it would break dio semantic in ext2/3.
> > Currently in ext2/3 if we issue a overwrite dio and then issue a read
> > dio.  We will always read the latest data because we wait on i_mutex
> > lock.  But after parallelizing overwite dio, this semantic might breaks.
> > I re-read this doc but it seems that it doesn't describe this case.  Do
> > we need to keep this semantic?
>   I'm not sure but also I don't think it's important to optimize that
> special case.

Thanks for the comment.  I am really not sure whether it is worth.  Let
me test the performance w/ and w/o dioread_nolock first. :-)

Regards,
                                                - Zheng