Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk drivers

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

On 19/09/12 11:51, Oliver Chick wrote:
> This patch implements persistent grants for the xen-blk{front,back}
> mechanism.
[...]
> We (ijc, and myself) have introduced a new constant,
> BLKIF_MAX_PERS_REQUESTS_PER_DEV. This is to prevent a malicious guest
> from attempting a DoS, by supplying fresh grefs, causing the Dom0
> kernel from to map excessively. 
[...]
> 2) Otherwise, we revert to non-persistent grants for all future grefs.

Why fallback instead of immediately failing the request?

> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index 73f196c..f95dee9 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -78,6 +78,7 @@ struct pending_req {
>  	unsigned short		operation;
>  	int			status;
>  	struct list_head	free_list;
> +	u8			is_pers;

Using "pers" as an abbreviation for "persistent" isn't obvious.  For
readability it may be better spell it in full.

> +/*
> + * Maximum number of persistent grants that can be mapped by Dom0 for each
> + * interface. This is set to be the size of the ring, as this is a limit on
> + * the number of requests that can be inflight at any one time. 256 imposes
> + * an overhead of 11MB of mapped kernel space per interface.
> + */
> +#define BLKIF_MAX_PERS_REQUESTS_PER_DEV 256

This 11MB per VBD seems like a lot.  With 150 VMs each with 2 VBDs this
requires > 3 GB.  Is this a scalability problem?

Does there need to be a mechanism to expire old maps in blkback?

David