Re: [PATCH 3/7] quota: add new quotactl Q_GETNEXTQUOTA

Jan Kara
2016-01-22T09:28:17Z

On Thu 21-01-16 22:07:20, Eric Sandeen wrote:
> Q_GETNEXTQUOTA is exactly like Q_GETQUOTA, except that it
> will return quota information for the id equal to or greater
> than the id requested. In other words, if the requested id has
> no quota, the command will return quota information for the
> next higher id which does have a quota set. If no higher id
> has an active quota, -ESRCH is returned.
>
> This allows filesystems to do efficient iteration in kernelspace,
> much like extN filesystems do in userspace when asked to report
> all active quotas.
>
> This does require a new data structure for userspace, as the
> current structure does not include an ID for the returned quota
> information.
>
> Today, Ext4 with a hidden quota inode requires getpwent-style
> iterations, and for systems which have i.e. LDAP backends,
> this can be very slow, or even impossible if iteration is not
> allowed in the configuration.
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>

Same comments as for XFS version apply here. Furthermore:

> diff --git a/fs/quota/compat.c b/fs/quota/compat.c
> index fb1892f..80773a4 100644
> --- a/fs/quota/compat.c
> +++ b/fs/quota/compat.c
> @@ -19,6 +19,19 @@ struct compat_if_dqblk {
> compat_uint_t dqb_valid;
> };
>
> +struct compat_if_nextdqblk {
> + compat_u64 dqb_bhardlimit;
> + compat_u64 dqb_bsoftlimit;
> + compat_u64 dqb_curspace;
> + compat_u64 dqb_ihardlimit;
> + compat_u64 dqb_isoftlimit;
> + compat_u64 dqb_curinodes;
> + compat_u64 dqb_btime;
> + compat_u64 dqb_itime;
> + compat_uint_t dqb_valid;
> + compat_uint_t dqb_id;
> +};
> +

Is there a need for compat version of this structure? Everything is
naturally aligned and the size is a multiple of 8 bytes. But these things
keep surprising me... Added CC to linux-api in a hope that there's someone
who definitely knows.

On a side note when I have API people attention: AFAIU we need
struct compat_if_dqblk (defined in fs/quota/compat.c) because its size is
not multiple of 8 bytes, right? But what is then then reason for get_user()
/ put_user() when the structure is copied into userspace (see below)?
copy_in_user() should have copied everything...

if (copy_in_user(compat_dqblk, dqblk, sizeof(*compat_dqblk)) ||
get_user(data, &dqblk->dqb_valid) ||
put_user(data, &compat_dqblk->dqb_valid))
ret = -EFAULT;

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR