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

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Date:   Wed, 23 Oct 2019 12:01:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Bobrowski <mbobrowski@...browski.org>
Cc:     Jan Kara <jack@...e.cz>, "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 Wed 23-10-19 13:35:19, Matthew Bobrowski wrote:
> 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.

Yes, that's what I'd suggest as well. In fact when doing that you can share
file_write_and_wait_range() call with the one already in ext4_sync_file()
use for other cases. Similarly with file_check_and_advance_wb_err(). So the
copied bit will be really only:

        ret = sync_mapping_buffers(inode->i_mapping);
        if (!(inode->i_state & I_DIRTY_ALL))
                goto out;
        if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
                goto out;

        err = sync_inode_metadata(inode, 1);
        if (ret == 0)
                ret = err;

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

Yes, please.

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