Re: [PATCH RFC v4 14/22] mm: page_reporting: skip redundant zeroing of host-zeroed reported pages

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

On 4/27/26 17:13, Zi Yan wrote:
> On Sun Apr 26, 2026 at 5:48 PM EDT, 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:
>>
>> - 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.
>>
>> - Add PG_zeroed page flag (sharing PG_private bit) to mark pages
>>   that have been zeroed by the host.  Set it on reported pages during
>>   allocation from the buddy in page_del_and_expand().
>>
>> - Thread the zeroed bool through rmqueue -> prep_new_page ->
>>   post_alloc_hook, where it skips redundant zeroing for __GFP_ZERO
>>   allocations.
>>
>> 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
>> Assisted-by: cursor-agent:GPT-5.4-xhigh
>> ---
>>  include/linux/page-flags.h     |  9 +++++
>>  include/linux/page_reporting.h |  3 ++
>>  mm/compaction.c                |  6 ++--
>>  mm/internal.h                  |  2 +-
>>  mm/page_alloc.c                | 66 +++++++++++++++++++++++-----------
>>  mm/page_reporting.c            | 14 +++++++-
>>  mm/page_reporting.h            | 12 +++++++
>>  7 files changed, 87 insertions(+), 25 deletions(-)
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index f7a0e4af0c73..eef2499cba8b 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -135,6 +135,8 @@ enum pageflags {
>>  	PG_swapcache = PG_owner_priv_1, /* Swap page: swp_entry_t in private */
>>  	/* Some filesystems */
>>  	PG_checked = PG_owner_priv_1,
>> +	/* Page contents are known to be zero */
>> +	PG_zeroed = PG_private,
> 
> +willy,
> 
> I was discussing with willy and David about removing PG_private and
> repurposing it to PG_folio to identify folios. IIUC, PG_zeroed is only
> set for PageBuddy, so it should not be an issue to set it for allocated
> pages for folio identification. Let me know if I get it wrong.

Right, we can keep using that flag here. Whenever we leave the buddy it gets
cleared. So as part of your work, simply let the buddy use the renamed flag.

-- 
Cheers,

David