Re: [PATCH RFC 2/9] mm: page_reporting: skip redundant zeroing of host-zeroed reported pages

David Hildenbrand (Arm)
2026-04-13T08:01:08Z

On 4/13/26 00:50, Michael S. Tsirkin wrote:
> When a guest reports free pages to the hypervisor via the page reporting
> framework (used by virtio-balloon and hv_balloon), the host typically
> zeros those pages when reclaiming their backing memory. However, when
> those pages are later allocated in the guest, post_alloc_hook()
> unconditionally zeros them again if __GFP_ZERO is set. This
> double-zeroing is wasteful, especially for large pages.
>
> Avoid redundant zeroing by propagating the "host already zeroed this"
> information through the allocation path:
>
> 1. Add a host_zeroes_pages flag to page_reporting_dev_info, allowing
> drivers to declare that their host zeros reported pages on reclaim.
> A static key (page_reporting_host_zeroes) gates the fast path.
>
> 2. In page_del_and_expand(), when the page was reported and the
> static key is enabled, stash a sentinel value (MAGIC_PAGE_ZEROED)
> in page->private.
>
> 3. In post_alloc_hook(), check page->private for the sentinel. If
> present and zeroing was requested (but not tag zeroing), skip
> kernel_init_pages().
>
> In particular, __GFP_ZERO is used by the x86 arch override of
> vma_alloc_zeroed_movable_folio.
>
> No driver sets host_zeroes_pages yet; a follow-up patch to
> virtio_balloon is needed to opt in.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Assisted-by: Claude:claude-opus-4-6
> ---
> include/linux/mm.h | 6 ++++++
> include/linux/page_reporting.h | 3 +++
> mm/page_alloc.c | 21 +++++++++++++++++++++
> mm/page_reporting.c | 9 +++++++++
> mm/page_reporting.h | 2 ++
> 5 files changed, 41 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5be3d8a8f806..59fc77c4c90e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4814,6 +4814,12 @@ static inline bool user_alloc_needs_zeroing(void)
> &init_on_alloc);
> }
>
> +/*
> + * Sentinel stored in page->private to indicate the page was pre-zeroed
> + * by the hypervisor (via free page reporting).
> + */
> +#define MAGIC_PAGE_ZEROED 0x5A45524FU /* ZERO */

Why are we not using another page flag that is yet unused for buddy pages?

Using page->private for that, and exposing it to buddy users with the
__GFP_PREZEROED flag (I hope we can avoid that) does not sound
particularly elegant.

Also, if we're going to remember that some pages in the buddy are
pre-zeroed, it should better not be free-page-reporting specific.

I'd assume ordinary inflating+deflating of the balloon would also end up
with pre-zeroed pages. We'd just need a (mm/balloon.c -specific)
interface to tell the buddy that the pages are zeroed.

--
Cheers,

David