linux-kernel - Re: your patch "mm: Remove false WARN_ON from pagecache_isize

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Message-ID: <20141103221849.GB23575@dastard>
Date:	Tue, 4 Nov 2014 09:18:49 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Jan Kara <jack@...e.cz>, xfs@....sgi.com,
	linux-kernel@...r.kernel.org
Subject: Re: your patch "mm: Remove false WARN_ON from
 pagecache_isize_extended()"

On Mon, Nov 03, 2014 at 04:41:13PM +0000, Jan Beulich wrote:
> Jan,
> 
> having run into that warning too, I looked into it a little, and now
> having found that patch am pretty uncertain: Both truncate_setsize()
> and pagecache_isize_extended() document that they want to be
> called with i_mutex held, so removing the WARN_ON() alone seems
> either incomplete or wrong. What I found to work without violating
> this documented requirement is the patch below.

Or, just perhaps, the comments are wrong....

Some filesystems have stronger, more robust internal serialisation
than the VFS provides with i_mutex....

> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -797,7 +797,7 @@ xfs_file_fallocate(
>  		     FALLOC_FL_COLLAPSE_RANGE | FALLOC_FL_ZERO_RANGE))
>  		return -EOPNOTSUPP;
>  
> -	xfs_ilock(ip, XFS_IOLOCK_EXCL);
> +	xfs_rw_ilock(ip, XFS_IOLOCK_EXCL);
>  	if (mode & FALLOC_FL_PUNCH_HOLE) {
>  		error = xfs_free_file_space(ip, offset, len);
>  		if (error)

The i_mutex is completely redundant here. Not to mention there are
multiple callers of these xfs_*_file_space() functions used to
implement fallocate(), and we're not about to change the locking
model for them....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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