linux-kernel - Re: [PATCH v2 7/7] virtio_balloon: Stop calling page_address() in free

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Message-ID: <aK9Ogjn71JoOM3w3@fedora>
Date: Wed, 27 Aug 2025 11:29:22 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2 7/7] virtio_balloon: Stop calling page_address() in
 free_pages()

On Wed, Aug 27, 2025 at 02:01:01PM +0200, David Hildenbrand wrote:
> On 26.08.25 22:56, Vishal Moola (Oracle) wrote:
> > free_pages() should be used when we only have a virtual address. We
> > should call __free_pages() directly on our page instead.
> > 
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> > ---
> >   drivers/virtio/virtio_balloon.c | 3 +--
> >   1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index eae65136cdfb..d4e6865ce355 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -488,8 +488,7 @@ static unsigned long return_free_pages_to_mm(struct virtio_balloon *vb,
> >   		page = balloon_page_pop(&vb->free_page_list);
> >   		if (!page)
> >   			break;
> > -		free_pages((unsigned long)page_address(page),
> > -			   VIRTIO_BALLOON_HINT_BLOCK_ORDER);
> > +		__free_pages(page, VIRTIO_BALLOON_HINT_BLOCK_ORDER);
> >   	}
> >   	vb->num_free_page_blocks -= num_returned;
> >   	spin_unlock_irq(&vb->free_page_list_lock);
> 
> I think you missed another nastiness of similar kind in
> get_free_page_and_send() where we do
> 
> 	p = page_address(page);
> 
> Just to call
> 
> 	free_pages((unsigned long)p, VIRTIO_BALLOON_HINT_BLOCK_ORDER);

Thanks for catching that. Andrew can you fold the attached patch into
this one please? It looks like the page_address() call is needed for other
things, but since we're changing the file we might as well clean these
up as well.

I imagine theres more of these lingering in the kernel, but theres so
many callers and I only looked for the ones that were calling
page_address() inline :(.

View attachment "0001-virtio_ballon-Call-__free_pages-in-get_free_page_and.patch" of type "text/plain" (1415 bytes)