From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH 12/14] xen-blkback: safely unmap grants in
 case they are still in use
Date: Fri, 23 Jan 2015 16:00:43 +0000
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To: =?windows-1252?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>, David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>, Jenny Herbert <jennifer.herbert@citrix.com>
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On 23/01/15 15:47, Roger Pau Monn=E9 wrote:
> El 23/01/15 a les 15.54, David Vrabel ha escrit:
>> On 23/01/15 14:31, Roger Pau Monn=E9 wrote:
>>> El 19/01/15 a les 16.51, David Vrabel ha escrit:
>>>> +		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;
>>>> +			xen_blkbk_unmap_done(blkif, unmap_pages, invcount);
>>>>  		}
>>>> -	}
>>>> -	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;
> =

> This should be fixed to at least be (which is still not fully correct,
> but it's better):
> =

> pages +=3D invcount;
> num -=3D invcount;
> =

> I hope an example will clarify this, suppose we have the following pages
> array:
> =

> pages[0] =3D persistent grant
> pages[1] =3D persistent grant
> pages[2] =3D regular grant
> pages[3] =3D persistent grant
> pages[4] =3D regular grant
> =

> And batch is 1. In this case, the unmapped grant will be pages[2], but
> then due to the code below pages will be updated to point to &pages[1],
> which has already been scanned. If this was done correctly pages should
> point to &pages[3]. As said, it's not really a bug, but the loop is
> sub-optimal.

Ah ha.  Thanks for the clear explanation.

gnttab_blkback_unmap_prepare() stops once its been through the whole
batch regardless of whether it filled the array with ops so we don't
check a page twice but this does mean we have a sub-optimal number of ops.

David