Re: [PATCHv4 12/14] xen-blkback: safely unmap grants in case they are still in use

["Roger Pau Monné" <roger.pau@citrix.com>]

El 26/01/15 a les 18.01, David Vrabel ha escrit:
> From: Jennifer Herbert <jennifer.herbert@citrix.com>
> 
> Use gnttab_unmap_refs_async() to wait until the mapped pages are no
> longer in use before unmapping them.
> 
> This allows blkback to use network storage which may retain refs to
> pages in queued skbs after the block I/O has completed.
> 
> Signed-off-by: Jennifer Herbert <jennifer.herbert@citrix.com>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

This looks fine, only one comment below.

[...]

> +/*
> + * Unmap the grant references, and also remove the M2P over-rides
> + * used in the 'pending_req'.
> + */
> +static void xen_blkbk_unmap(struct xen_blkif *blkif,
> +                            struct grant_page *pages[],
> +                            int num)
> +{
> +	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct page *unmap_pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	unsigned int invcount = 0;
> +	int ret;
> +
> +	while (num) {
> +		unsigned int batch = min(num, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +		
> +		invcount = xen_blkbk_unmap_prepare(blkif, pages, batch,
> +						   unmap, unmap_pages);
> +		if (invcount) {
> +			ret = gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
>  			BUG_ON(ret);
>  			put_free_pages(blkif, unmap_pages, invcount);
> -			invcount = 0;
>  		}
> -	}
> -	if (invcount) {
> -		ret = gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
> -		BUG_ON(ret);
> -		put_free_pages(blkif, unmap_pages, invcount);
> +		pages += batch;
> +		num -= batch;

Could you add a comment explaining that the loop is not optimal if the
pages array contains both persistent and non-persistent grants?

Roger.