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

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Date:	Wed, 31 Jul 2013 23:35:11 -0700
From:	Matt Wilson <msw@...zon.com>
To:	Roger Pau Monne <roger.pau@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [PATCH 3/3] xen-blkfront: revoke foreign access for grants not
 mapped by the backend

On Wed, Jul 31, 2013 at 05:00:44PM +0200, 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@...rix.com>

Acked-by: Matt Wilson <msw@...zon.com>

> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
>  drivers/block/xen-blkfront.c |   33 +++++++++++++++++++++++++++++----
>  1 files changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 187a437..bc9fc7d 100644
> --- 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);
> +		}
>  	}
>  	if (s->req.operation == BLKIF_OP_INDIRECT) {
>  		for (i = 0; i < INDIRECT_GREFS(nseg); i++) {
> -			list_add(&s->indirect_grants[i]->node, &info->persistent_gnts);
> -			info->persistent_gnts_c++;
> +			if (gnttab_query_foreign_access(s->indirect_grants[i]->gref)) {
> +				list_add(&s->indirect_grants[i]->node, &info->persistent_gnts);
> +				info->persistent_gnts_c++;
> +			} else {
> +				gnttab_end_foreign_access(s->indirect_grants[i]->gref, 0, 0UL);
> +				s->indirect_grants[i]->gref = GRANT_INVALID_REF;
> +				list_add_tail(&s->indirect_grants[i]->node,
> +				              &info->persistent_gnts);
> +			}
>  		}
>  	}
>  }
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