Re: [RFC PATCH] Btrfs-progs: disable qgroupid 0 for quota_tree

On Mon, 12 Nov 2012 07:25:08 +0100, Jan Schmidt wrote:

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In kernel, qgroupid 0 is a special number when we run the quota group
limit command.

So, we should not be able to create a quota group whose id is 0,
otherwise the kernel can't deal with it. Fix it.

This is probably a stupid question - but if its not meant to be possible
to create such a thing shouldn't this be fixed in the kernel (as well as
here) to reject attempts from user space to create it?

Otherwise it's possible for a non-aware program (or a user who is
playing) to still create it.

Right. It also should be fixed in the kernel side, the patch is coming.

But since we know which number is valid or not, it is better that we also check
the arguments in the user tool before they are passed into the kernel. So, we
can avoid trapping into the kernel, which will waste time, and output the error
information as soon as possible.

Have the kernel return with errno EINVAL is the way everyone else (I know about)
does this.

I would avoid doing a check more than once where ever possible. Primarily,
because duplicating checks is error prone and in case of a kernel interface it
complicates future changes.

You've got a point there. I'll send out the kernel patch.

Thanks
Miao