Re: XFS fiemap issue with Linux 3.10.0-327.el7.x86_64 (CentOS 7)

[Eric Sandeen <sandeen@sandeen.net>]

On 2/15/16 6:20 PM, Dave Chinner wrote:
> On Mon, Feb 15, 2016 at 04:47:16PM -0600, Eric Sandeen wrote:
>>
>>
>> On 2/15/16 2:40 PM, Dave Chinner wrote:
>>> On Mon, Feb 15, 2016 at 02:28:36PM -0500, Jim Wilcoxson wrote:
>>>> Thanks Eric.
>>>>
>>>> I ran xfs_bmap -v and it returned extents 0-19999, alternating data
>>>> with holes.  The holes and data were various sizes, I suppose for xfs
>>>> alignment reasons, but everything was there.
>>>>
>>>> Running fiemap again after xfs_bmap still returned 1364 extents.
>>>
>>> Yes, fiemap in XFS uses a buffer size of:
>>>
>>>         bm.bmv_count = min_t(__s32, bm.bmv_count,
>>>                              (PAGE_SIZE * 16 / sizeof(struct getbmapx)));
>>>
>>> i.e. limits a single fiemap fetch to a maximum of 64k of extent
>>> data.
>>>
>>> I think you have an incorrect assumption about fiemap behaviour.
>>> fiemap is not designed to return or even count all the extents in a
>>> file in a single call; 
>>
>> I think it is; as was quoted earlier,
>>
>> "If fm_extent_count is zero, then the
>> fm_extents[] array is ignored (no extents will be returned), and the
>> fm_mapped_extents count will hold the number of extents needed in
>> fm_extents[] to hold the file's current mapping."
>>
>> and that's in there:
>>
>>         /* only count the extents */
>>         if (fieinfo->fi_extents_max == 0) {
>>                 fieinfo->fi_extents_mapped++;
>>                 return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
>>         }
> 
> Right, but I think we only pass one /internal/ buffer's extents to
> the fill function. When we run out of extents to process (i.e we hit
> the limit in the above extent count passed to xfs_getbmapx), it
> returns to userspace.
> 
> This isn't a problem when iterating extent lists, because of the
> FIEMAP_EXTENT_LAST is exported to userspace. The problem appears to
> be that counting has different semantics, and I bet nobody wrote a
> regression test that covered this case....

Yeah, I looked at xfs_io's fiemap, and there's no way to pass it
a count-only set of options.

>>> on XFS it returns how many extents it can
>>> return in a single call. When you then map the file, if the
>>> FIEMAP_EXTENT_LAST is not set on the last extent returned, then the
>>> application needs to make another FIEMAP call from the end offset of
>>> last extent mapping returned in the last call, and it will then
>>> return the next N extents in the file.
>>>
>>> IOWs, you have to keep calling FIEMAP to map the entire file, not
>>> assume a single call will return an arbitrary amount of data to you
>>> in a single call.
>>
>> He's not trying to get all data in one call, just a count.
> 
> I was under the impression that counting was a ranged operation,
> too. i.e. you can ask for the number of extents within a certain
> file offset. Maybe we have got counting wrong as FIEMAP_EXTENT_LAST
> is never exposed to userspace, nor does the fiemap code record the
> offset we counted up to, so it would be hard to iterate.
> 
> Patches to fix the kernel, to support extent counting in xfs_io
> and a regression test, please!

Yep....

-Eric

> Cheers,
> 
> Dave.
> 

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