Re: [Xen-devel] [PATCH] xen-blkfront: use old rinfo after enomem during migration

[Manjunath Patil <manjunath.b.patil@oracle.com>]

On 11/30/2018 2:33 PM, Boris Ostrovsky wrote:

> On 11/30/18 4:49 PM, Manjunath Patil wrote:
>> Thank you Boris for your comments. I removed faulty email of mine.
>>
>> replies inline.
>> On 11/30/2018 12:42 PM, Boris Ostrovsky wrote:
>>> On 11/29/18 12:17 AM, Manjunath Patil wrote:
>>>> Hi,
>>>> Feel free to suggest/comment on this.
>>>>
>>>> I am trying to do the following at dst during the migration now.
>>>> 1. Dont clear the old rinfo in blkif_free(). Instead just clean it.
>>>> 2. Store the old rinfo and nr_rings into temp variables in
>>>> negotiate_mq()
>>>> 3. let nr_rings get re-calculated based on backend data
>>>> 4. try allocating new memory based on new nr_rings
>>> Since I suspect number of rings will likely be the same why not reuse
>>> the rings in the common case?
>> I thought attaching devices will be more often than migration. Hence
>> did not want add to an extra check for
>>    - if I am inside migration code path and
>>    - if new nr_rings is equal to old nr_rings or not
>>
>> Sure addition of such a thing would avoid the memory allocation
>> altogether in migration path,
>> but it would add a little overhead for normal device addition.
>>
>> Do you think its worth adding that change?
>
> IMO a couple of extra checks are not going to make much difference.
I will add this change
>
> I wonder though --- have you actually seen the case where you did fail
> allocation and changes provided in this patch made things work? I am
> asking because right after negotiate_mq() we will call setup_blkring()
> and it will want to allocate bunch of memory. A failure there is fatal
> (to ring setup). So it seems to me that you will survive negotiate_mq()
> but then will likely fail soon after.
I have noticed the ENOMEM insise negotiate_mq() on ct machine. When I 
included my patch, I manually triggered the ENOMEM using a debug flag.
The patch works for ENOMEM inside negotiate_mq().

As you mentioned, if we really hit the ENOMEM in negotiate_mq(), we 
might hit it in setup_blkring() as well.
We should add the similar change to blkif_sring struct as well.

I will make this change as well and send the new patch-set for review.
>
>>>
>>>> 5.
>>>>     a. If memory allocation is a success
>>>>        - free the old rinfo and proceed to use the new rinfo
>>>>     b. If memory allocation is a failure
>>>>        - use the old the rinfo
>>>>        - adjust the nr_rings to the lowest of new nr_rings and old
>>>> nr_rings
>>>> @@ -1918,10 +1936,24 @@ static int negotiate_mq(struct blkfront_info
>>>> *info)
>>>>                      sizeof(struct blkfront_ring_info),
>>>>                      GFP_KERNEL);
>>>>        if (!info->rinfo) {
>>>> -        xenbus_dev_fatal(info->xbdev, -ENOMEM, "allocating
>>>> ring_info structure");
>>>> -        info->nr_rings = 0;
>>>> -        return -ENOMEM;
>>>> -    }
>>>> +        if (unlikely(nr_rings_old)) {
>>>> +            /* We might waste some memory if
>>>> +             * info->nr_rings < nr_rings_old
>>>> +             */
>>>> +            info->rinfo = rinfo_old;
>>>> +            if (info->nr_rings > nr_rings_old)
>>>> +                info->nr_rings = nr_rings_old;
>>>> +            xenbus_dev_fatal(info->xbdev, -ENOMEM,
>>> Why xenbus_dev_fatal()?
>> I wanted to make sure that this msg is seen on console by default. So
>> that we know there was a enomem event happened and we recovered from it.
>> What do you suggest instead? xenbus_dev_error?
> Neither. xenbus_dev_fatal() is going to change connection state so it is
> certainly not what we want. And even xenbus_dev_error() doesn't look
> like the right thing to do since as far as block device setup is
> concerned there are no errors.
>
> Maybe pr_warn().
I will include this.

Thank you for your comments.
>
> -boris
>
>
>>> -boris
>>>
>>>
>>>> +            "reusing old ring_info structure(new ring size=%d)",
>>>> +                info->nr_rings);
>>>> +        } else {
>>>> +            xenbus_dev_fatal(info->xbdev, -ENOMEM,
>>>> +                "allocating ring_info structure");
>>>> +            info->nr_rings = 0;
>>>> +            return -ENOMEM;
>>>> +        }
>>>> +    } else if (unlikely(nr_rings_old))
>>>> +        kfree(rinfo_old);
>>>>          for (i = 0; i < info->nr_rings; i++) {
>>>>            struct blkfront_ring_info *rinfo;
>
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