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? 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
