[Xen-devel] Re: [PATCH 3/3] mm: Extend memory hotplug API to allow memory hotplug in virtual machines

[Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>]

On Mon, 2011-03-28 at 11:25 +0200, Daniel Kiper wrote:
> This patch contains online_page_chain and apropriate functions
> for registering/unregistering online page notifiers. It allows
> to do some machine specific tasks during online page stage which
> is required to implement memory hotplug in virtual machines.
> Additionally, __online_page_increment_counters() and
> __online_page_free() function was add to ease generic
> hotplug operation.

I really like that you added some symbolic constants there.  It makes it
potentially a lot more readable.

My worry is that the next person who comes along is going to _really_
scratch their head asking why they would use:
OP_DO_NOT_INCREMENT_TOTAL_COUNTERS or: OP_INCREMENT_TOTAL_COUNTERS.
There aren't any code comments about it, and the patch description
doesn't really help.

In the end, we're only talking about a couple of lines of code for each
case (reordering the function a bit too):

        void online_page(struct page *page)
        {
        // 1. pfn-based bits upping the max physical address markers:
                unsigned long pfn = page_to_pfn(page);
                if (pfn >= num_physpages)
                        num_physpages = pfn + 1;
        #ifdef CONFIG_FLATMEM
                max_mapnr = max(page_to_pfn(page), max_mapnr);
        #endif
        
        // 2. number of pages counters:
                totalram_pages++;
        #ifdef CONFIG_HIGHMEM
                if (PageHighMem(page))
                        totalhigh_pages++;
        #endif
        
        // 3. preparing 'struct page' and freeing:
                ClearPageReserved(page);
                init_page_count(page);
                __free_page(page);
        }
        
Your stuff already extracted the free stuff very nicely.  I think now we
just need to separate out the totalram_pages/totalhigh_pages bits from
the num_physpages/max_mapnr ones.

If done right, this should also help the totalram_pages/totalhigh_pages
go away balloon_retrieve(), and make Xen less likely to break in the
future.  It also makes it immediately obvious why Xen skips incrementing
those counters: it does it later.

I also note that Xen has a copy of a part of online_page() in its
increase_reservation(): 

                /* Relinquish the page back to the allocator. */
                ClearPageReserved(page);
                init_page_count(page);
                __free_page(page);

That means that Xen is basically carrying an open-coded copy of
online_page() all by itself today.  

-- Dave


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