[Xen-devel] Re: [PATCH R4 7/7] xen/balloon: Memory hotplug support for Xen balloon driver

[Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>]

On Tue, 2011-03-08 at 22:50 +0100, Daniel Kiper wrote:
> +static enum bp_state reserve_additional_memory(long credit)
> +{
> +     int rc;
> +     unsigned long balloon_hotplug = credit;
> +
> +     balloon_hotplug <<= PAGE_SHIFT;
> +
> +     rc = add_virtual_memory((u64 *)&balloon_hotplug);

This would work if all 'unsigned long's were 64-bits.  It'll break on
32-bit kernels in a very bad way by overwriting 4 bytes of stack.

> +     if (rc) {
> +             pr_info("xen_balloon: %s: add_virtual_memory() failed: %i\n", 
> __func__, rc);
> +             return BP_EAGAIN;
> +     }
> +
> +     balloon_hotplug >>= PAGE_SHIFT;
> +
> +     balloon_hotplug -= credit;
> +
> +     balloon_stats.hotplug_pages += credit;
> +     balloon_stats.balloon_hotplug = balloon_hotplug;
> +
> +     return BP_DONE;
> +}
> +
> +static int xen_online_page_notifier(struct notifier_block *nb, unsigned long 
> val, void *v)
> +{
> +     struct page *page = v;
> +     unsigned long pfn = page_to_pfn(page);
> +
> +     if (pfn >= num_physpages)
> +             num_physpages = pfn + 1;
> +
> +     inc_totalhigh_pages();
> +
> +#ifdef CONFIG_FLATMEM
> +     max_mapnr = max(pfn, max_mapnr);
> +#endif

I really don't like that this is a direct copy of online_page() up to
this point.  They're already subtly different.  I'm also curious if this
breaks on 32-bit kernels because of the unconditional
inc_totalhigh_pages().

If it's done this way, I'd almost guarantee that the first time someone
fixes a bug or adds a generic feature in online_page() that Xen gets
missed.  

> +     mutex_lock(&balloon_mutex);
> +
> +     __balloon_append(page);
> +
> +     if (balloon_stats.hotplug_pages)
> +             --balloon_stats.hotplug_pages;
> +     else
> +             --balloon_stats.balloon_hotplug;
> +
> +     mutex_unlock(&balloon_mutex);
> +
> +     return NOTIFY_STOP;
> +}

I'm not a _huge_ fan of these notifier chains, but I guess it works.
However, if you're going to use these notifier chains, then we probably
should use them to full effect.  Have a notifier list like this:

        1. generic online_page()
        2. xen_online_page_notifier() (returns NOTIFY_STOP)
        3. free_online_page()

Where finish_online_page() does something like this:

finish_online_page(...)
{
        ClearPageReserved(page);
        init_page_count(page);
        __free_page(page);
}

These patches are definitely getting there.  Just another round or two,
and they should be ready to go.

-- Dave


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