[PATCH v2] XFS: Let the broken fiemap work in query mode.

[Tao Ma]

Dave Chinner wrote:
> On Wed, Apr 28, 2010 at 10:00:01AM +0800, Tao Ma wrote:
>> Hi Dave,
>>
>> Dave Chinner wrote:
>>> On Tue, Apr 27, 2010 at 02:17:45PM +0800, Tao Ma wrote:
>>>> According to Documentation/filesystems/fiemap.txt, 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.
>>>>
>>>> But as the commit 97db39a1f6f69e906e98118392400de5217aa33a has changed
>>>> bmv_count to the caller's input buffer, this number query function can't
>>>> work any more. As this commit is written to change bmv_count from
>>>> MAXEXTNUM because of ENOMEM, we can't find a really suitable number to
>>>> set bmv_count now in xfs_vn_fiemap. Since we really have no idea of how
>>>> much extents the file has, a big number may cause ENOMEM, while a small
>>>> one will mask the real extent no.
>>>>
>>>> So this patch try to resolve this problem by adding a temporary getbmapx
>>>> in xfs_getbmap. If the caller didn't give bmv_count, we don't allocate
>>>> the "out" either. Instead, every time we want to use 'out', use '&tmp'
>>>> instead.
>>>>
>>>> I know this solution is a bit ugly, but I can't find a way to resolve
>>>> this issue while not changing the codes too much. So any good suggestion
>>>> is welcomed.
>>> I don't see a need to change xfs_getbmap() to fix this. We can limit
>>> the maximum allocation size to something realistic just by setting
>>> bm.bmv.count to something sane. e.g, in xfs_vn_fiemap:
>>>
>>> -	bm.bmv_count = fieinfo->fi_extents_max + 1;
>>> +	bm.bmv.count = !fieinfo->fi_extents_max ? MAXEXTNUM :
>>> +					fieinfo->fi_extents_max - 1;
>>> +	bm.bmv_count = MIN(bm.bmv_count,
>>> 				(PAGE_SIZE * 16 / sizeof(struct getbmapx)));
>>>
>>> Unless I'm missing something, that should also prevent the case of
>>> an application providing a really large fi_extents_max from
>>> triggering ENOMEM in most cases as well.
>> I just worry about one thing: What if the real extent number is
>> larger than the PAGE_SIZE * 16 / sizeof(struct getbmapx)? In this
>> case, we will give up the wrong extent number to the user space.
> 
> Applications need to handle mappings changing from query to getting
> the mapping, so this should not be a major issue. Especially as the
> method of fiemap indicating there are more extents to be extracted
> from the inode in the case the kernel can't allocate a buffer large
> enough is already documented.
> 
> Realistically though, xfs_getbmap() needs a complete rewrite so
> right now I'd prefer just to do the minimum to fix the reported
> problem that continue to make it into even more of a mess than it is
> now...
Fair enough. Here is the updated patch.

btw, I am working on adding the test cases in xfstests.

Regards,
Tao