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

[Michael S. Tsirkin]

On Mon, Apr 13, 2026 at 10:00:58AM +0200, David Hildenbrand (Arm) wrote:
> 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?

Because we need to report the status *after* it left buddy.
And all flags are in use at that point.


> 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.

But propagating this all over mm does not sound too palatable, right?
There's precedent with MAGIC_HWPOISON already.
Better ideas? Thanks!

> 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.
> 

Indeed, it's also easily possible - it's a separate optimization, though.
Another simple enhancement is including hugetlbfs freelists in page
reporting.
Doesn't need to block this patchset though, right?

> 
> -- 
> Cheers,
> 
> David