Re: [Qemu-devel] [PULL 23/26] virtio-balloon: Safely handle BALLOON_PAGE_SIZE < host page size

[David Gibson]

On Tue, Mar 05, 2019 at 04:06:54PM +0000, Peter Maydell wrote:
> On Fri, 22 Feb 2019 at 02:41, Michael S. Tsirkin <address@hidden> wrote:
> >
> > From: David Gibson <address@hidden>
> >
> > The virtio-balloon always works in units of 4kiB (BALLOON_PAGE_SIZE), but
> > we can only actually discard memory in units of the host page size.
> 
> Hi -- Coverity points out an issue in this patch (CID 1399146):
> 
> > +    /* Hard case
> > +     *
> > +     * We've put a piece of a larger host page into the balloon - we
> > +     * need to keep track until we have a whole host page to
> > +     * discard
> > +     */
> > +    warn_report_once(
> > +"Balloon used with backing page size > 4kiB, this may not be reliable");
> > +
> > +    subpages = rb_page_size / BALLOON_PAGE_SIZE;
> > +
> > +    if (balloon->pbp
> > +        && (rb != balloon->pbp->rb
> > +            || host_page_base != balloon->pbp->base)) {
> > +        /* We've partially ballooned part of a host page, but now
> > +         * we're trying to balloon part of a different one.  Too hard,
> > +         * give up on the old partial page */
> > +        free(balloon->pbp);
> > +        balloon->pbp = NULL;
> >      }
> >
> > -    ram_block_discard_range(rb, ram_offset, rb_page_size);
> > -    /* We ignore errors from ram_block_discard_range(), because it has
> > -     * already reported them, and failing to discard a balloon page is
> > -     * not fatal */
> > +    if (!balloon->pbp) {
> > +        /* Starting on a new host page */
> > +        size_t bitlen = BITS_TO_LONGS(subpages) * sizeof(unsigned long);
> > +        balloon->pbp = g_malloc0(sizeof(PartiallyBalloonedPage) + bitlen);
> 
> 
> We allocate balloon->pbp with g_malloc0() here...
> 
> > +        balloon->pbp->rb = rb;
> > +        balloon->pbp->base = host_page_base;
> > +    }
> > +
> > +    bitmap_set(balloon->pbp->bitmap,
> > +               (ram_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
> > +               subpages);
> > +
> > +    if (bitmap_full(balloon->pbp->bitmap, subpages)) {
> > +        /* We've accumulated a full host page, we can actually discard
> > +         * it now */
> > +
> > +        ram_block_discard_range(rb, balloon->pbp->base, rb_page_size);
> > +        /* We ignore errors from ram_block_discard_range(), because it
> > +         * has already reported them, and failing to discard a balloon
> > +         * page is not fatal */
> > +
> > +        free(balloon->pbp);
> 
> ...but we free it (here and elsewhere) with free(), not g_free().

Ah.  Whoops.

I'll put a fix for that in the series of followup balloon patches I'm
working on right now.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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