FYI, I noticed this new patch submitted upstream.

If it gets included upstream, there is a patch forward to allow the
standard quota tools to query Lustre quota data. 

Cheers, Andreas

Begin forwarded message:

> From: Sascha Hauer <s.hauer@pengutronix.de>
> Date: January 22, 2021 at 07:17:45 PST
> To: linux-fsdevel@vger.kernel.org
> Cc: Richard Weinberger <richard@nod.at>, linux-mtd@lists.infradead.org, kernel@pengutronix.de, Jan Kara <jack@suse.com>, Sascha Hauer <s.hauer@pengutronix.de>
> Subject: [PATCH 1/8] quota: Allow to pass mount path to quotactl
> 
> This patch introduces the Q_PATH flag to the quotactl cmd argument.
> When given, the path given in the special argument to quotactl will
> be the mount path where the filesystem is mounted, instead of a path
> to the block device.
> This is necessary for filesystems which do not have a block device as
> backing store. Particularly this is done for upcoming UBIFS support.
> 
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> ---
> fs/quota/quota.c           | 66 ++++++++++++++++++++++++++++----------
> include/uapi/linux/quota.h |  1 +
> 2 files changed, 50 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/quota/quota.c b/fs/quota/quota.c
> index 6d16b2be5ac4..f653b27a9a4e 100644
> --- a/fs/quota/quota.c
> +++ b/fs/quota/quota.c
> @@ -17,6 +17,7 @@
> #include <linux/capability.h>
> #include <linux/quotaops.h>
> #include <linux/types.h>
> +#include <linux/mount.h>
> #include <linux/writeback.h>
> #include <linux/nospec.h>
> #include "compat.h"
> @@ -859,25 +860,10 @@ static bool quotactl_cmd_onoff(int cmd)
>         (cmd == Q_XQUOTAON) || (cmd == Q_XQUOTAOFF);
> }
> 
> -/*
> - * look up a superblock on which quota ops will be performed
> - * - use the name of a block device to find the superblock thereon
> - */
> -static struct super_block *quotactl_block(const char __user *special, int cmd)
> +static struct super_block *quotactl_sb(dev_t dev, int cmd)
> {
> -#ifdef CONFIG_BLOCK
>    struct super_block *sb;
> -    struct filename *tmp = getname(special);
>    bool excl = false, thawed = false;
> -    int error;
> -    dev_t dev;
> -
> -    if (IS_ERR(tmp))
> -        return ERR_CAST(tmp);
> -    error = lookup_bdev(tmp->name, &dev);
> -    putname(tmp);
> -    if (error)
> -        return ERR_PTR(error);
> 
>    if (quotactl_cmd_onoff(cmd)) {
>        excl = true;
> @@ -901,12 +887,50 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
>        goto retry;
>    }
>    return sb;
> +}
> +
> +/*
> + * look up a superblock on which quota ops will be performed
> + * - use the name of a block device to find the superblock thereon
> + */
> +static struct super_block *quotactl_block(const char __user *special, int cmd)
> +{
> +#ifdef CONFIG_BLOCK
> +    struct filename *tmp = getname(special);
> +    int error;
> +    dev_t dev;
> 
> +    if (IS_ERR(tmp))
> +        return ERR_CAST(tmp);
> +    error = lookup_bdev(tmp->name, &dev);
> +    putname(tmp);
> +    if (error)
> +        return ERR_PTR(error);
> +
> +    return quotactl_sb(dev, cmd);
> #else
>    return ERR_PTR(-ENODEV);
> #endif
> }
> 
> +static struct super_block *quotactl_path(const char __user *special, int cmd)
> +{
> +    struct super_block *sb;
> +    struct path path;
> +    int error;
> +
> +    error = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
> +               &path);
> +    if (error)
> +        return ERR_PTR(error);
> +
> +    sb = quotactl_sb(path.mnt->mnt_sb->s_dev, cmd);
> +
> +    path_put(&path);
> +
> +    return sb;
> +}
> +
> /*
>  * This is the system call interface. This communicates with
>  * the user-level programs. Currently this only supports diskquota
> @@ -920,6 +944,7 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
>    struct super_block *sb = NULL;
>    struct path path, *pathp = NULL;
>    int ret;
> +    bool q_path;
> 
>    cmds = cmd >> SUBCMDSHIFT;
>    type = cmd & SUBCMDMASK;
> @@ -927,6 +952,9 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
>    if (type >= MAXQUOTAS)
>        return -EINVAL;
> 
> +    q_path = cmds & Q_PATH;
> +    cmds &= ~Q_PATH;
> +
>    /*
>     * As a special case Q_SYNC can be called without a specific device.
>     * It will iterate all superblocks that have quota enabled and call
> @@ -951,7 +979,11 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
>            pathp = &path;
>    }
> 
> -    sb = quotactl_block(special, cmds);
> +    if (q_path)
> +        sb = quotactl_path(special, cmds);
> +    else
> +        sb = quotactl_block(special, cmds);
> +
>    if (IS_ERR(sb)) {
>        ret = PTR_ERR(sb);
>        goto out;
> diff --git a/include/uapi/linux/quota.h b/include/uapi/linux/quota.h
> index f17c9636a859..e1787c0df601 100644
> --- a/include/uapi/linux/quota.h
> +++ b/include/uapi/linux/quota.h
> @@ -71,6 +71,7 @@
> #define Q_GETQUOTA 0x800007    /* get user quota structure */
> #define Q_SETQUOTA 0x800008    /* set user quota structure */
> #define Q_GETNEXTQUOTA 0x800009    /* get disk limits and usage >= ID */
> +#define Q_PATH     0x400000    /* quotactl special arg contains mount path */
> 
> /* Quota format type IDs */
> #define    QFMT_VFS_OLD 1
> -- 
> 2.20.1
>