On Thu, May 29, 2008 at 11:33:09AM -0400, jim owens wrote:
> Christoph Hellwig wrote:
>>
>> What use is there geeting the extent count for a range? I'd rather
>> do it only per-file like the xfs ioctl.
>
> I'll answer that from practical experience. Our api equivalents:
>
> __u64 fm_start; /* logical offset (inclusive) at
> * which to start mapping (in) */
> __u64 fm_length; /* logical length of mapping which
> * userspace cares about (in) */
> __u32 fm_extent_count; /* size of fm_extents array (in) */
> __u32 fm_mapped_extents; /* number of extents that were
> * mapped (out) */
>
> ... note it has no flags field and no separate ioctl_extent_count.
>
> "fm_extent_count" is
> IN == max_extents to return.
> OUT == number of extents remaining in-range after fm_mapped_extents
>
> Pass in fm_extent_count==0 and you get OUT number of extent entries
> within your fm_start + fm_length range. Which you can use to set
> your malloc size because the FS can have massive extent counts :(
See, that's a very natural API. I'd be much happier with that -
"extent_count == 0" is consistent with no extent structures passed in.
It also fits with many other API that do it the same way (eg, snprintf).
Joel
--
"I don't know anything about music. In my line you don't have
to."
- Elvis Presley
Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@xxxxxxxxxx
Phone: (650) 506-8127
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