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

[Manjunath Patil <manjunath.b.patil@xxxxxxxxxx>]

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?
> > 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?
> -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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