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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

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 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.


>>
>>
>>> 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().

-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;
>