[Xen-devel] Re: [PATCH R2 7/7] xen/balloon: Xen memory balloon driver with memory hotplug support

[Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>]

On Wed, 2010-12-29 at 18:07 +0100, Daniel Kiper wrote:
> +config XEN_BALLOON_MEMORY_HOTPLUG
> +     bool "Xen memory balloon driver with memory hotplug support"
> +     default n
> +     depends on XEN_BALLOON && MEMORY_HOTPLUG
> +     help
> +       Xen memory balloon driver with memory hotplug support allows expanding
> +       memory available for the system above limit declared at system 
> startup.
> +       It is very useful on critical systems which require long run without
> +       rebooting.

This might be better phrased as "Memory hotplug support for Xen balloon
driver".  It might otherwise confuse people about whether they're seeing
some kind of choice or an _enhancement_ to the existing driver.

Also, why bother even making this a config option?  What are the
downsides if it was always compiled in?  You could even make it a
non-prompting Kconfig option and just automatically turn it on with
XEN_BALLOON && MEMORY_HOTPLUG.


> +static int allocate_memory_resource(struct resource **r, unsigned long 
> nr_pages)
>  {
> -     unsigned long target = balloon_stats.target_pages;
> +     int rc;
> +     resource_size_t r_min, r_size;
> +
> +     /*
> +      * Look for first unused memory region starting at page
> +      * boundary. Skip last memory section created at boot time
> +      * becuase it may contains unused memory pages with PG_reserved
> +      * bit not set (online_pages require PG_reserved bit set).
> +      */
> +
> +     *r = kzalloc(sizeof(struct resource), GFP_KERNEL);
> 
> -     target = min(target,
> -                  balloon_stats.current_pages +
> -                  balloon_stats.balloon_low +
> -                  balloon_stats.balloon_high);
> +     if (!*r)
> +             return -ENOMEM;
> 
> -     return target;
> +     (*r)->name = "System RAM";
> +     (*r)->flags = IORESOURCE_MEM | IORESOURCE_BUSY;
> +     r_min = 
> PFN_PHYS(section_nr_to_pfn(pfn_to_section_nr(balloon_stats.boot_max_pfn) + 
> 1));
> +     r_size = nr_pages << PAGE_SHIFT;
> +
> +     rc = allocate_resource(&iomem_resource, *r, r_size, r_min,
> +                                     ULONG_MAX, PAGE_SIZE, NULL, NULL);
> +
> +     if (rc < 0) {
> +             kfree(*r);
> +             *r = NULL;
> +     }
> +
> +     return rc;
>  }

The double-pointer stuff here ends up looking a little funky.  Is there
any reason you don't just use ERR_PTRs?  That might look a bit more
sane.

> +static void adjust_memory_resource(struct resource **r, unsigned long 
> nr_pages)
> +{
> +     if ((*r)->end + 1 - (nr_pages << PAGE_SHIFT) == (*r)->start) {
> +             BUG_ON(release_resource(*r) < 0);

In some kernels, people do:

        #define BUG_ON(...) do{}while(0)

to save space.  If anyone ever does that with this code, it'll break
horribly.  It's also hard to read these.  So, please break logic actions
_out_ of the BUG_ON() arguments.

That's repeated in quite a few places in here.  Make sure to go get them
all.

It also isn't evident what this patch set is trying to do until you get
down to this 7/7 patch.  You might want to put a more complete
description in 0/7.

-- Dave


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