linux-ext4 - Re: [PATCH] Revert "ext4: use ext4_write_inode() when fsyncing w/o a journal"

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Date:   Fri, 1 Feb 2019 22:21:20 +0100
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>, jack@...e.cz
Subject: Re: [PATCH] Revert "ext4: use ext4_write_inode() when fsyncing w/o a
 journal"

On Thu 31-01-19 23:42:19, Theodore Ts'o wrote:
> This reverts commit ad211f3e94b314a910d4af03178a0b52a7d1ee0a.
> 
> As Jan Kara pointed out, this change was unsafe since it means we lose
> the call to sync_mapping_buffers() in the nojournal case.  The
> original point of the commit was avoid taking the inode mutex (since
> it causes a lockdep warning in generic/113); but we need the mutex in
> order to call sync_mapping_buffers().

Actually, I don't think sync_mapping_buffers() needs inode mutex (i_rwsem
these days). It uses blkdev_mapping->private_lock for synchronization of
operations on the list of buffers and fsync_buffers_list() seems to be
pretty careful about races with mark_buffer_dirty_inode(). So why do you
think we need i_rwsem?

> The real fix to this problem was discussed here:
> 
> https://lore.kernel.org/lkml/20181025150540.259281-4-bvanassche@acm.org
> 
> The proposed patch was to fix a syzbot complaint, but the problem can
> also demonstrated via "kvm-xfstests -c nojournal generic/113".
> Multiple solutions were discused in the e-mail thread, but none have
> landed in the kernel as of this writing.  Anyway, commit
> ad211f3e94b314 is absolutely the wrong way to suppress the lockdep, so
> revert it.
> 
> Fixes: ad211f3e94b314a910d4af03178a0b52a7d1ee0a ("ext4: use ext4_write_inode() when fsyncing w/o a journal")
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> Reported: Jan Kara <jack@...e.cz>

So if you decide to go via a safe way of reverting the change, I'm fine
with that so feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/ext4/fsync.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
> index 712f00995390..5508baa11bb6 100644
> --- a/fs/ext4/fsync.c
> +++ b/fs/ext4/fsync.c
> @@ -116,16 +116,8 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
>  		goto out;
>  	}
>  
> -	ret = file_write_and_wait_range(file, start, end);
> -	if (ret)
> -		return ret;
> -
>  	if (!journal) {
> -		struct writeback_control wbc = {
> -			.sync_mode = WB_SYNC_ALL
> -		};
> -
> -		ret = ext4_write_inode(inode, &wbc);
> +		ret = __generic_file_fsync(file, start, end, datasync);
>  		if (!ret)
>  			ret = ext4_sync_parent(inode);
>  		if (test_opt(inode->i_sb, BARRIER))
> @@ -133,6 +125,9 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
>  		goto out;
>  	}
>  
> +	ret = file_write_and_wait_range(file, start, end);
> +	if (ret)
> +		return ret;
>  	/*
>  	 * data=writeback,ordered:
>  	 *  The caller's filemap_fdatawrite()/wait will sync the data.
> -- 
> 2.19.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR