RE: [PATCH 2/2] Drivers: hv: Add Hyper-V balloon driver

KY Srinivasan
2012-10-10T00:19:05Z

> -----Original Message-----
> From: Andrew Morton [mailto:akpm@linux-foundation.org]
> Sent: Tuesday, October 09, 2012 3:45 PM
> To: KY Srinivasan
> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> andi@firstfloor.org
> Subject: Re: [PATCH 2/2] Drivers: hv: Add Hyper-V balloon driver
>
> On Sun, 7 Oct 2012 16:59:46 -0700
> "K. Y. Srinivasan" <kys@microsoft.com> wrote:
>
> > Add the basic balloon driver.
>
> hm, how many balloon drivers does one kernel need?
>
> Although I see that the great majority of this code is hypervisor-specific.
>
> > Windows hosts dynamically manage the guest
> > memory allocation via a combination memory hot add and ballooning. Memory
> > hot add is used to grow the guest memory upto the maximum memory that can
> be
> > allocatted to the guest. Ballooning is used to both shrink as well as expand
> > up to the max memory. Supporting hot add needs additional support from the
> > host. We will support hot add when this support is available. For now,
> > by setting the VM startup memory to the VM max memory, we can use
> > ballooning alone to dynamically manage memory allocation amongst
> > competing guests on a given host.
> >
> >
> > ...
> >
> > +static int alloc_balloon_pages(struct hv_dynmem_device *dm, int
> num_pages,
> > + struct dm_balloon_response *bl_resp, int alloc_unit,
> > + bool *alloc_error)
> > +{
> > + int i = 0;
> > + struct page *pg;
> > +
> > + if (num_pages < alloc_unit)
> > + return 0;
> > +
> > + for (i = 0; (i * alloc_unit) < num_pages; i++) {
> > + if (bl_resp->hdr.size + sizeof(union dm_mem_page_range) >
> > + PAGE_SIZE)
> > + return i * alloc_unit;
> > +
> > + pg = alloc_pages(GFP_HIGHUSER | __GFP_NORETRY |
> GFP_ATOMIC |
> > + __GFP_NOMEMALLOC | __GFP_NOWARN,
> > + get_order(alloc_unit << PAGE_SHIFT));
>
> This choice of GFP flags is basically impossible to understand, so I
> suggest that a comment be added explaining it all.
>
> I'm a bit surprised at the inclusion of GFP_ATOMIC as it will a) dip
> into page reserves, whcih might be undesirable and b) won't even
> reclaim clean pages, which seems desirable. I suggest this also be
> covered in the forthcoming code comment.

I will rework these flags and add appropriate comments.

>
> drivers/misc/vmw_balloon.c seems to me to have used better choices here.
>
> > + if (!pg) {
> > + *alloc_error = true;
> > + return i * alloc_unit;
> > + }
> > +
> > + totalram_pages -= alloc_unit;
>
> Well, I'd consider totalram_pages to be an mm-private thing which drivers
> shouldn't muck with. Why is this done?

By modifying the totalram_pages, the information presented in /proc/meminfo
correctly reflects what is currently assigned to the guest (MemTotal).

>
> drivers/xen/balloon.c and drivers/virtio/virtio_balloon.c also alter
> totalram_pages, also without explaining why.
> drivers/misc/vmw_balloon.c does not.
>
> > + dm->num_pages_ballooned += alloc_unit;
> > +
> > + bl_resp->range_count++;
> > + bl_resp->range_array[i].finfo.start_page =
> > + page_to_pfn(pg);
> > + bl_resp->range_array[i].finfo.page_cnt = alloc_unit;
> > + bl_resp->hdr.size += sizeof(union dm_mem_page_range);
> > +
> > + }
> > +
> > + return num_pages;
> > +}
> >
> > ...
> >
>
>
>

Thanks for the prompt review. I will address your comments and repost the patches soon.
If it is ok with you, I am going to keep the code that manipulates totalram_pages
(for reasons I listed above).

Regards,

K. Y