linux-ext4 - Re: [PATCH v5 00/12] ext4: port direct I/O to iomap infrastructure

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Message-ID: <20191023023519.GA16505@bobrowski>
Date:   Wed, 23 Oct 2019 13:35:19 +1100
From:   Matthew Bobrowski <mbobrowski@...browski.org>
To:     Jan Kara <jack@...e.cz>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, david@...morbit.com, darrick.wong@...cle.com
Subject: Re: [PATCH v5 00/12] ext4: port direct I/O to iomap infrastructure

On Mon, Oct 21, 2019 at 09:43:30PM +0200, Jan Kara wrote:
> On Mon 21-10-19 09:31:12, Theodore Y. Ts'o wrote:
> > Hi Matthew, thanks for your work on this patch series!
> > 
> > I applied it against 4c3, and ran a quick test run on it, and found
> > the following locking problem.  To reproduce:
> > 
> > kvm-xfstests -c nojournal generic/113
> > 
> > generic/113		[09:27:19][    5.841937] run fstests generic/113 at 2019-10-21 09:27:19
> > [    7.959477] 
> > [    7.959798] ============================================
> > [    7.960518] WARNING: possible recursive locking detected
> > [    7.961225] 5.4.0-rc3-xfstests-00012-g7fe6ea084e48 #1238 Not tainted
> > [    7.961991] --------------------------------------------
> > [    7.962569] aio-stress/1516 is trying to acquire lock:
> > [    7.963129] ffff9fd4791148c8 (&sb->s_type->i_mutex_key#12){++++}, at: __generic_file_fsync+0x3e/0xb0
> > [    7.964109] 
> > [    7.964109] but task is already holding lock:
> > [    7.964740] ffff9fd4791148c8 (&sb->s_type->i_mutex_key#12){++++}, at: ext4_dio_write_iter+0x15b/0x430
> 
> This is going to be a tricky one. With iomap, the inode locking is handled
> by the filesystem while calling generic_write_sync() is done by
> iomap_dio_rw(). I would really prefer to avoid tweaking iomap_dio_rw() not
> to call generic_write_sync(). So we need to remove inode_lock from
> __generic_file_fsync() (used from ext4_sync_file()). This locking is mostly
> for legacy purposes and we don't need this in ext4 AFAICT - but removing
> the lock from __generic_file_fsync() would mean auditing all legacy
> filesystems that use this to make sure flushing inode & its metadata buffer
> list while it is possibly changing cannot result in something unexpected. I
> don't want to clutter this series with it so we are left with
> reimplementing __generic_file_fsync() inside ext4 without inode_lock. Not
> too bad but not great either. Thoughts?

So, I just looked at this on my lunch break and I think the simplest approach
would be to just transfer the necessary chunks of code from within
__generic_file_fsync() into ext4_sync_file() for !journal cases, minus the
inode lock, and minus calling into __generic_file_fsync(). I don't forsee this
causing any issues, but feel free to correct me if I'm wrong.

If this is deemed to be OK, then I will go ahead and include this as a
separate patch in my series.

--<M>--