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