Re: [for-6.16-final PATCH 8/9] nfs/localio: avoid bouncing LOCALIO if nfs_client_is_local()

[Jeff Layton <jlayton@kernel.org>]

On Sun, 2025-07-13 at 23:13 -0400, Mike Snitzer wrote:
> From: Mike Snitzer <snitzer@hammerspace.com>
> 
> Previously nfs_local_probe() was made to disable and then attempt to
> re-enable LOCALIO (via LOCALIO protocol handshake) if/when it was
> called and LOCALIO already enabled.
> 
> Vague memory for _why_ this was the case is that this was useful
> if/when a local NFS server were to be restarted with a local NFS
> client connected to it.
> 
> But as it happens this causes an absurd amount of LOCALIO flapping
> which has a side-effect of too much IO being needlessly sent to NFSD
> (using RPC over the loopback network interface).  This is the
> definition of "serious performance loss" (that negates the point of
> having LOCALIO).
> 
> So remove this mis-optimization for re-enabling LOCALIO if/when an NFS
> server is restarted (which is an extremely rare thing to do).  Will
> revisit testing that scenario again but in the meantime this patch
> restores the full benefit of LOCALIO.
> 
> Fixes: 56bcd0f07fdb ("nfs: implement client support for NFS_LOCALIO_PROGRAM")
> Signed-off-by: Mike Snitzer <snitzer@hammerspace.com>
> ---
>  fs/nfs/localio.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
> index a4bacd9a5052..e3ae003118cb 100644
> --- a/fs/nfs/localio.c
> +++ b/fs/nfs/localio.c
> @@ -180,10 +180,8 @@ static void nfs_local_probe(struct nfs_client *clp)
>  		return;
>  	}
>  
> -	if (nfs_client_is_local(clp)) {
> -		/* If already enabled, disable and re-enable */
> -		nfs_localio_disable_client(clp);
> -	}
> +	if (nfs_client_is_local(clp))
> +		return;
>  
>  	if (!nfs_uuid_begin(&clp->cl_uuid))
>  		return;
> @@ -241,7 +239,8 @@ __nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
>  		case -ENOMEM:
>  		case -ENXIO:
>  		case -ENOENT:
> -			/* Revalidate localio, will disable if unsupported */
> +			/* Revalidate localio */
> +			nfs_localio_disable_client(clp);
>  			nfs_local_probe(clp);
>  		}
>  	}

Reviewed-by: Jeff Layton <jlayton@kernel.org>