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

[David Hildenbrand (Arm)]

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@xxxxxxxxxx>
> 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