linux-kernel - Re: [PATCH] xen/blkfront: correct purging of persistent grants

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Date:   Fri, 28 Sep 2018 10:03:28 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org
Cc:     roger.pau@...rix.com, konrad.wilk@...cle.com, axboe@...nel.dk
Subject: Re: [PATCH] xen/blkfront: correct purging of persistent grants

On 9/28/18 9:52 AM, Juergen Gross wrote:
> 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@...e.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.

I am not arguing with correctness of your patch, so

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>

but I am a little surprised that it fixes Sander's problem.


-boris


>
> 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