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

["David Hildenbrand (Arm)" <david@kernel.org>]

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 <mst@redhat.com>
>>> 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