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. The comment above MAGIC_PAGE_ZEROED triggered my reaction. -- Cheers, David
