Re: [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context

From: Josef Bacik <josef@toxicpanda.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
	Dave Chinner <david@fromorbit.com>,
	David Sterba <dsterba@suse.com>,
	"linux-btrfs @ vger . kernel . org" <linux-btrfs@vger.kernel.org>,
	Filipe Manana <fdmanana@gmail.com>,
	Christoph Hellwig <hch@lst.de>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context
Date: Wed, 2 Sep 2020 12:47:28 -0400	[thread overview]
Message-ID: <d001c8df-1c35-6426-4000-ad1222af7196@toxicpanda.com> (raw)
In-Reply-To: <20200902162944.GH6090@magnolia>

On 9/2/20 12:29 PM, Darrick J. Wong wrote:
> On Wed, Sep 02, 2020 at 07:10:08AM -0400, Josef Bacik wrote:
>> On 9/2/20 3:12 AM, Johannes Thumshirn wrote:
>>> On 02/09/2020 02:22, Josef Bacik wrote:
>>>> Instead now we have to rip
>>>> it out until we figure out what to do about it.
>>>
>>> I don't think we need to rip out the iomap conversion. We can
>>> take my fix albeit not pretty, until we have reworked the locking
>>> around ->fsync(). Probably with a big fat comment attached to it.
>>>
>>
>> We do, because your fix breaks DSYNC for AIO.  You didn't hit this with
>> direct io, you hit it with AIO, and the reason you hit it is because you are
>> on zram, so your bio's completed before we exited iomap_dio_rw.  So that was
>> the last put on the iomap_dio, and thus we ran
>> iomap_dio_complete() and deadlocked.  We can't just drop the DSYNC thing for
>> AIO because in the normal case where this doesn't happen we need to know
>> when the last thing is finished in order to run ->fsync(), we can't just run
>> it after submission.  Thanks,
> 
> Bleh, Oracle mail (or vger or something) is being slow again...
> 
> It occurred to me that we added iomap_dio_ops.submit_io for the benefit
> of btrfs.  Could we solve all this for now by adding a ->write_sync
> function pointer to iomap_dio_ops that could lead back into a btrfs
> function that would flush the necessary bits without itself taking the
> inode lock?  And if a ->write_sync is not supplied, then the caller gets
> generic_write_sync?
> 
> It's kind of a bandaid, but maybe less bad of one than restructuring the
> btrfs locking model under time pressure...
> 

I'd rather not mess around with the generic iomap stuff for this, coordinating 
changes between generic and fs stuff is annoying enough as it is.  We've got a 
strategy to work around this in btrfs so we don't have to rip out the iomap work 
right now.  And then we'll rip out the workaround once we've reworked the 
locking, since the locking stuff will require a fair bit of testing and soak 
time to be sure it's safe.  Thanks,

Josef