Re: [Xen-devel] [PATCH RFC 3/4] xen-blkfront: prevent hoarding all grants

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 08/07/13 14:03, Roger Pau Monne wrote:
> Prevent blkfront from hoarding all grants by adding a minimum number
> of grants that must be free at all times. We still need a way to free
> unused grants in blkfront, but this patch will mitigate the problem
> in the meantime.

I think this could end up with the a frontend being able to get no
grants because some other frontend always grabs the free grants first.

Particularly as the free grant callbacks are done in the wrong order.

David

> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -101,6 +101,12 @@ MODULE_PARM_DESC(max, "Maximum amount of segments in 
> indirect requests (default
>  #define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
>  
>  /*
> + * Make sure there are at least MIN_FREE_GRANTS at all times, preventing
> + * blkfront from hoarding all grants
> + */
> +#define MIN_FREE_GRANTS              512
> +
> +/*
>   * We have one of these per vbd, whether ide, scsi or 'other'.  They
>   * hang in private_data off the gendisk structure. We may end up
>   * putting all kinds of interesting stuff here :-)
> @@ -412,13 +418,13 @@ static int blkif_queue_request(struct request *req)
>       if (info->persistent_gnts_c < max_grefs) {
>               new_persistent_gnts = 1;
>               if (gnttab_alloc_grant_references(
> -                 max_grefs - info->persistent_gnts_c,
> +                 max_grefs - info->persistent_gnts_c + MIN_FREE_GRANTS,
>                   &gref_head) < 0) {
>                       gnttab_request_free_callback(
>                               &info->callback,
>                               blkif_restart_queue_callback,
>                               info,
> -                             max_grefs);
> +                             max_grefs + MIN_FREE_GRANTS);
>                       return 1;
>               }
>       } else


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