Re: [PATCH RFC 02/24] filelock: add a lm_set_conflict lease_manager callback

[Chuck Lever]

On Fri, Mar 15, 2024 at 12:52:53PM -0400, Jeff Layton wrote:
> The NFSv4.1 protocol adds support for directory delegations, but it
> specifies that if you already have a delegation and try to request a new
> one on the same filehandle, the server must reply that the delegation is
> unavailable.
> 
> Add a new lease_manager callback to allow the lease manager (nfsd in
> this case) to impose extra checks when performing a setlease.
> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/locks.c               |  5 +++++
>  include/linux/filelock.h | 10 ++++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index cb4b35d26162..415cca8e9565 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1822,6 +1822,11 @@ generic_add_lease(struct file *filp, int arg, struct file_lease **flp, void **pr
>  			continue;
>  		}
>  
> +		/* Allow the lease manager to veto the setlease */
> +		if (lease->fl_lmops->lm_set_conflict &&
> +		    lease->fl_lmops->lm_set_conflict(lease, fl))
> +			goto out;
> +
>  		/*
>  		 * No exclusive leases if someone else has a lease on
>  		 * this file:
> diff --git a/include/linux/filelock.h b/include/linux/filelock.h
> index daee999d05f3..c5fc768087df 100644
> --- a/include/linux/filelock.h
> +++ b/include/linux/filelock.h
> @@ -49,6 +49,16 @@ struct lease_manager_operations {
>  	int (*lm_change)(struct file_lease *, int, struct list_head *);
>  	void (*lm_setup)(struct file_lease *, void **);
>  	bool (*lm_breaker_owns_lease)(struct file_lease *);
> +
> +	/**
> +	 * lm_set_conflict - extra conditions for setlease
> +	 * @new: new file_lease being set
> +	 * @old: old (extant) file_lease
> +	 *
> +	 * This allows the lease manager to add extra conditions when
> +	 * setting a lease.

To make it clear which return value causes add_lease() to abort, I'd
rather see API contract-style descriptions of the meaning of the
return values instead of this design note. Something like:

 * Return values:
 *   %true: @new and @old conflict
 *   %false: No conflict detected


> +	 */
> +	bool (*lm_set_conflict)(struct file_lease *new, struct file_lease *old);
>  };
>  
>  struct lock_manager {
> 
> -- 
> 2.44.0
> 

-- 
Chuck Lever