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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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?


> 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()?

-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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