Re: [PATCH v2 3/3] pnfs: add a new mechanism to select a layout driver according to an ordered list

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Jun 09, 2016 at 08:21:05AM -0400, Jeff Layton wrote:
> Currently, the layout driver selection code always chooses the first one
> from the list. That's not really ideal however, as the server can send
> the list of layout types in any order that it likes. It's up to the
> client to select the best one for its needs.
> 
> This patch adds an ordered list of preferred driver types and has the
> selection code sort the list of avaiable layout drivers according to it.
> Any unrecognized layout type is sorted to the end of the list.
> 
> For now, the order of preference is hardcoded, but it should be possible
> to make this configurable in the future.
> 
> Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
> ---
>  fs/nfs/pnfs.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 50 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
> index 2fc1b9354345..c31d30f1e5f2 100644
> --- a/fs/nfs/pnfs.c
> +++ b/fs/nfs/pnfs.c
> @@ -99,6 +99,21 @@ unset_pnfs_layoutdriver(struct nfs_server *nfss)
>  }
>  
>  /*
> + * When the server sends a list of layout types, we choose one in the order
> + * given in the list below.
> + *
> + * FIXME: should this list be configurable via module_param or mount option?
> + */
> +static const u32 ld_prefs[] = {
> +	LAYOUT_SCSI,
> +	LAYOUT_BLOCK_VOLUME,
> +	LAYOUT_OSD2_OBJECTS,
> +	LAYOUT_FLEX_FILES,
> +	LAYOUT_NFSV4_1_FILES,
> +	0
> +};
> +
> +/*
>   * Try to set the server's pnfs module to the pnfs layout type specified by id.
>   * Currently only one pNFS layout driver per filesystem is supported.
>   *
> @@ -110,6 +125,8 @@ set_pnfs_layoutdriver(struct nfs_server *server, const struct nfs_fh *mntfh,
>  {
>  	struct pnfs_layoutdriver_type *ld_type = NULL;
>  	u32 id;
> +	int i, j;
> +	bool swapped;
>  
>  	if (!(server->nfs_client->cl_exchange_flags &
>  		 (EXCHGID4_FLAG_USE_NON_PNFS | EXCHGID4_FLAG_USE_PNFS_MDS))) {
> @@ -118,18 +135,44 @@ set_pnfs_layoutdriver(struct nfs_server *server, const struct nfs_fh *mntfh,
>  		goto out_no_driver;
>  	}
>  
> -	id = ids[0];
> -	if (!id)
> -		goto out_no_driver;
> +	/* bubble sort into ld_prefs order */
> +	do {
> +		swapped = false;
> +		for (i = 0; i < (NFS_MAX_LAYOUT_TYPES - 1); i++) {
>  
> -	ld_type = find_pnfs_driver(id);
> -	if (!ld_type) {
> -		request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX, id);
> +			/* If either is zero then we're done with this pass */
> +			if (ids[i] == 0 || ids[i + 1] == 0)
> +				break;
> +
> +			for (j = 0; ld_prefs[j] != 0; j++) {
> +				/* If we match left entry first, no swap */
> +				if (ids[i] == ld_prefs[j])
> +					break;
> +
> +				/* if we match the right, we need to swap */
> +				if (ids[i + 1] == ld_prefs[j]) {
> +					swap(ids[i], ids[i + 1]);
> +					swapped = true;
> +					break;
> +				}
> +			}
> +		}
> +	} while (swapped);

Maybe I'm strange, but, yes, to me this is easier to read as on a first
pass I can just take your word that the above is doing a sort and see
much more quickly what's happening....

Anyway, ACK to the series from me.

--b.

> +
> +	for (i = 0; i < NFS_MAX_LAYOUT_TYPES && ids[i] != 0; i++) {
> +		id = ids[i];
>  		ld_type = find_pnfs_driver(id);
> +		if (!ld_type) {
> +			request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX,
> +					id);
> +			ld_type = find_pnfs_driver(id);
> +		}
> +		if (ld_type)
> +			break;
>  	}
>  
>  	if (!ld_type) {
> -		dprintk("%s: No pNFS module found for %u.\n", __func__, id);
> +		dprintk("%s: No pNFS module found!\n", __func__);
>  		goto out_no_driver;
>  	}
>  
> -- 
> 2.5.5