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Subject: Re: [PATCH] xen/blkfront: correct purging of persistent grants
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-block@vger.kernel.org
Cc:     roger.pau@citrix.com, konrad.wilk@oracle.com, axboe@kernel.dk
References: <20180928072827.14229-1-jgross@suse.com>
 <95e32835-426f-36c9-a86a-09563f090f22@oracle.com>
 <772c3886-d4fe-b912-b125-a648b31a3074@suse.com>
 <b454645f-de15-2912-864e-18f00736013d@oracle.com>
From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 28 Sep 2018 15:52:56 +0200
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On 28/09/2018 15:33, Boris Ostrovsky wrote:
> On 9/28/18 9:13 AM, Juergen Gross wrote:
>> On 28/09/2018 14:45, Boris Ostrovsky wrote:
>>> On 9/28/18 3:28 AM, Juergen Gross wrote:
>>>> Commit a46b53672b2c2e3770b38a4abf90d16364d2584b ("xen/blkfront: cleanup
>>>> stale persistent grants") introduced a regression as purged persistent
>>>> grants were not pu into the list of free grants again. Correct that.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>> ---
>>>>  drivers/block/xen-blkfront.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>>>> index a71d817e900d..429d20131c7e 100644
>>>> --- a/drivers/block/xen-blkfront.c
>>>> +++ b/drivers/block/xen-blkfront.c
>>>> @@ -2670,8 +2670,8 @@ static void purge_persistent_grants(struct blkfront_info *info)
>>>>  			list_del(&gnt_list_entry->node);
>>>>  			gnttab_end_foreign_access(gnt_list_entry->gref, 0, 0UL);
>>>>  			rinfo->persistent_gnts_c--;
>>>> -			__free_page(gnt_list_entry->page);
>>>> -			kfree(gnt_list_entry);
>>>> +			gnt_list_entry->gref = GRANT_INVALID_REF;
>>>> +			list_add_tail(&gnt_list_entry->node, &rinfo->grants);
>>> Sorry, I don't follow this. What is the purpose of removing the grant
>>> from rinfo->grants list with list_del() and then adding it back with
>>> list_add_tail()?
>> The persistent grants are at the list head and the non-persistent ones
>> at the tail.
> 
> Oh, I didn't realize that. But isn't that an optimization (and so not
> following this rule should not cause errors)?

In theory: yes.

The persistent grant handling is rather complicated so I'd like to make
sure not to deviate from the common standards.

When I find some time I want to modify the persistent grant handling to
be more explicit and controlled completely by the frontend (within the
backend defined limits, of course). Until then we should try to modify
persistent grants not too much IMO.


Juergen