From mboxrd@z Thu Jan  1 00:00:00 1970
From: =?windows-1252?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>
Subject: Re: [PATCHv4 12/14] xen-blkback: safely unmap grants in
 case they are still in use
Date: Mon, 26 Jan 2015 20:21:34 +0100
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To: David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>, Jennifer Herbert <jennifer.herbert@citrix.com>
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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 Monn=E9 <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 =3D 0;
> +	int ret;
> +
> +	while (num) {
> +		unsigned int batch =3D min(num, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +		=

> +		invcount =3D xen_blkbk_unmap_prepare(blkif, pages, batch,
> +						   unmap, unmap_pages);
> +		if (invcount) {
> +			ret =3D gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
>  			BUG_ON(ret);
>  			put_free_pages(blkif, unmap_pages, invcount);
> -			invcount =3D 0;
>  		}
> -	}
> -	if (invcount) {
> -		ret =3D gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
> -		BUG_ON(ret);
> -		put_free_pages(blkif, unmap_pages, invcount);
> +		pages +=3D batch;
> +		num -=3D 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.