Re: [PATCH v10 24/37] mm: add put_page_zeroed and folio_put_zeroed

[David Hildenbrand (Arm)]

On 6/8/26 14:25, Lorenzo Stoakes wrote:
> On Mon, Jun 08, 2026 at 04:38:54AM -0400, Michael S. Tsirkin wrote:
>> Add put_page_zeroed() / folio_put_zeroed() for callers that hold
>> a reference to a page known to be zeroed.
>>
>> If this drops the last reference, the zeroed hint is
>> propagated to the buddy allocator.  If someone else still holds a
>> reference, the hint is simply lost - this is best-effort.
>>
>> This is useful for balloon drivers during deflation: the host
>> has already zeroed the pages, and the balloon is typically the
>> sole owner.  But if the page happens to be shared, silently
>> dropping the hint is safe and avoids the need for callers to
>> check the refcount.
>>
>> Note: put_page_zeroed uses folio_put_testzero() which only
>> detects sole ownership at the instant of the atomic decrement.
>> A concurrent reference holder (e.g. migration) means the hint
>> is silently lost. This is by design: the zeroed hint is a
>> performance optimization, not a correctness requirement.
>> Losing it just means the next allocation re-zeroes the page.
> 
> Do not put comments about specific expected races like this in the commit
> message but not in the code. Subtleties need to be called out.
> 
> The commit message also doesn't at all explain why PG_zeroed doesn't
> suffice here.
> 
>>
>> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> Assisted-by: Claude:claude-opus-4-6
> 
> I really don't understand why you have a 'zeroed' folio flag but need to
> also have new API calls to detect that?
> 
> They're also HORRIBLY named. Zeroed as in what? Zero page? Huge zero page?
> Memory zeroed by kernel? Pages that userland happen to have zeroed? Or host
> VM zeroed?
> 
> Each are cases we address individually and relate to folios.
> 
> You absolutely fail to clarify _which one_ you mean, and provide absolutely
> no documentation and add an exported mm API with no description.
> 
> This is just I think not something we want to add? Especially on something
> so fundamental?

I raised previously that providing a folio helper is odd, and that I suggested
that we defer this change.

Maybe we'd want to add such an interface for frozen pages later (to be used by
the balloon), but I don't think we want that for folios.

[1] https://lore.kernel.org/all/5f76af6e-9818-42ea-a305-c0fc1d920dca@xxxxxxxxxx/

-- 
Cheers,

David