Re: [Xen-devel] [PATCH 3/3] xen-blkfront: revoke foreign access for grants not mapped by the backend

[David Vrabel <david.vrabel@citrix.com>]

On 31/07/13 16:00, Roger Pau Monne wrote:
> There's no need to keep the foreign access in a grant if it is not
> persistently mapped by the backend. This allows us to free grants that
> are not mapped by the backend, thus preventing blkfront from hoarding
> all grants.
> 
> The main effect of this is that blkfront will only persistently map
> the same grants as the backend, and it will always try to use grants
> that are already mapped by the backend. Also the number of persistent
> grants in blkfront is the same as in blkback (and is controlled by the
> value in blkback).
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>

Reviewed-by: David Vrabel <david.vrabel@citrix.com>

But please see the documentation updates needed below.

> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -1016,13 +1016,38 @@ static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
>  	}
>  	/* Add the persistent grant into the list of free grants */
>  	for (i = 0; i < nseg; i++) {
> -		list_add(&s->grants_used[i]->node, &info->persistent_gnts);
> -		info->persistent_gnts_c++;
> +		if (gnttab_query_foreign_access(s->grants_used[i]->gref)) {
> +			/*
> +			 * If the grant is still mapped by the backend (the
> +			 * backend has chosen to make this grant persistent)
> +			 * we add it at the head of the list, so it will be
> +			 * reused first.
> +			 */
> +			list_add(&s->grants_used[i]->node, &info->persistent_gnts);
> +			info->persistent_gnts_c++;
> +		} else {
> +			/*
> +			 * If the grant is not mapped by the backend we end the
> +			 * foreign access and add it to the tail of the list,
> +			 * so it will not be picked again unless we run out of
> +			 * persistent grants.
> +			 */
> +			gnttab_end_foreign_access(s->grants_used[i]->gref, 0, 0UL);
> +			s->grants_used[i]->gref = GRANT_INVALID_REF;
> +			list_add_tail(&s->grants_used[i]->node, &info->persistent_gnts);
> +		}

Because you only reclaim grants when a request is completed, can you add
text similar to the following to xen/include/public/io/blkif.h (and the
Linux copy).

feature-persistent:
...
   When the backend driver needs to unmap a persistent grant it should
   do so prior to completing a request that used that grant reference.
   If a persistent grant is unmapped at any other time, the frontend
   driver may not notice and may be unable to reclaim the grant
   reference.

Thanks.

David