Re: [PATCH] xfs: Don't trim file extents during iomap

[Nikolay Borisov <nborisov@suse.com>]

On 28.11.2017 03:47, Darrick J. Wong wrote:
> On Mon, Nov 27, 2017 at 09:44:34AM -0800, Darrick J. Wong wrote:
>> On Thu, Nov 23, 2017 at 01:40:03PM +0200, Nikolay Borisov wrote:
>>> For file extents xfs currently calls xfs_bmapi_read with flag set to 0,
>>> meaning extents are going to be truncated to the requested range. Since the
>>> same codepath is used for fiemap this means xfs is special in that regard, since
>>> other filesystems (ext4/btrfs) do not trim extents for fiemap. Make the behavior
>>> consistent by always passing XFS_BMAPI_ENTIRE. A full xfstest run validated that
>>> this doesn't regress on ordinary read/write IO.
>>>
>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>>
>> Looks ok, will also test...
>> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> 
> ...and now withdrawn.
> 
> Looking at iomap_apply, it seems we call ->iomap_begin, and at line 73 we
> trim an iomap that is longer than the range we requested:
> 
> /*
>  * Cut down the length to the one actually provided by the filesystem,
>  * as it might not be able to give us the whole size that we requested.
>  */
> if (iomap.offset + iomap.length < pos + length)
> 	length = iomap.offset + iomap.length - pos;

This actually trims the requested length to match what iomap has given
is. I.e if we request a range between [50, 100] (length 50, offset 50)
but iomap returns

[40,60] (offset 40, length 20) then the trimmer range is :

[50, 10]. SO it's actually trimming the requested range against what was
actually returned by the filesystem not the other way around.


> 
> However, we do not trim the beginning off an iomap that starts before
> the 'pos' we passed to ->iomap_begin, so we're left with an iomap that
> can begin before the range.
> 
> FWIW I ran xfstests (like I told you to on IRC) and saw regressions
> in a whole bunch of tests:
> 
> generic/170 generic/287 generic/295 generic/326 generic/330 generic/333
> generic/372 xfs/214 xfs/237 xfs/249 xfs/258 xfs/346 xfs/420 xfs/421
> 
> From a brief glance it looks as though most of the iomap_apply'ers
> try to trim the iomap before using it, but clearly there's something
> wrong here.
> 
> --D
> 
>>
>>> ---
>>>  fs/xfs/xfs_iomap.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
>>> index f179bdf1644d..8942324a4d3d 100644
>>> --- a/fs/xfs/xfs_iomap.c
>>> +++ b/fs/xfs/xfs_iomap.c
>>> @@ -1008,7 +1008,7 @@ xfs_file_iomap_begin(
>>>  	end_fsb = XFS_B_TO_FSB(mp, offset + length);
>>>  
>>>  	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
>>> -			       &nimaps, 0);
>>> +			       &nimaps, XFS_BMAPI_ENTIRE);
>>>  	if (error)
>>>  		goto out_unlock;
>>>  
>>> -- 
>>> 2.7.4
>>>
>>> --
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