On Mon, Apr 13, 2026 at 10:15:08AM +0200, David Hildenbrand (Arm) wrote: > On 4/13/26 10:10, Michael S. Tsirkin wrote: > > 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 <[email protected]> > >>> 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. > > I'll comment on that on the other patch, where __GFP_PREZEROED, which I > really hate, is added. > > > > > > >> 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! > > I'll comment on the __GFP_PREZEROED patch. > > > > >> 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? > > Not blocking, but I don't want something that is too coupled to > free-page reporting optimizations in the buddy.
I can add that in the next version if you like, sure. The main issue is that it means we need a flag that survives free. And the benefit is much smaller - unlike page reports, deflates are rare. > The comment above > MAGIC_PAGE_ZEROED triggered my reaction. yea, that's more confusing than helpful. > -- > Cheers, > > David
