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

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Date:   Mon, 21 Oct 2019 21:43:30 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Matthew Bobrowski <mbobrowski@...browski.org>, jack@...e.cz,
        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 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?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR