Re: [PATCH v2 1/5] add metadata_incore ioctl in vfs

[Arnd Bergmann <arnd@xxxxxxxx>]

On Wednesday 05 January 2011 03:17:16 Shaohua Li wrote:
> On Tue, 2011-01-04 at 17:40 +0800, Arnd Bergmann wrote:
> 
> > Have you tried passing just a single metadata_incore_ent
> > at the ioctl and looping in user space? I would guess the
> > extra overhead of that would be small enough, but that might
> > need to be measured.
> metadata usually isn't continuous, so this means we have a lot of
> metadata_incore_ent entries. And this is called at boot time and I want
> to make the overhead as low as possible to not impact boot. Unless there
> are certain reasons we can't use indirect pointers, I'd like to make
> kernel return a vector of entries.

It's not a strict rule, but the indirect data passing is rather
ugly and I'd only do that if the difference can be /measured/.

If the purpose is to speed up boot time by preloading metadata,
the FIMETADATA_INCORE operations should of course not take a
significant amount of time compared to the actual preloading,
but as long as it's less than one percent of the time you need
for the preload, I would just use the simpler interface.

> @@ -882,6 +882,7 @@ COMPATIBLE_IOCTL(FIGETBSZ)
>  /* 'X' - originally XFS but some now in the VFS */
>  COMPATIBLE_IOCTL(FIFREEZE)
>  COMPATIBLE_IOCTL(FITHAW)
> +COMPATIBLE_IOCTL(FIMETADATA_INCORE)
>  COMPATIBLE_IOCTL(KDGETKEYCODE)
>  COMPATIBLE_IOCTL(KDSETKEYCODE)
>  COMPATIBLE_IOCTL(KDGKBTYPE)

This change can go away as well.

Two more general comments:

- You probably want to add the ioctls to file_ioctl instead of do_vfs_ioctl,
  so you don't add another case statement to the common path.

- I don't know if there are any rules for what should be an ioctl or an
  fcntl, we're rather inconsistent about this. If you have found a good
  reason for making it an ioctl, just put that into the changelog so we
  can refer to it next time.

	Arnd
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