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. > 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
