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. Signed-off-by: Michael S. Tsirkin <[email protected]> Assisted-by: Claude:claude-opus-4-6 --- include/linux/mm.h | 13 +++++++++++++ mm/swap.c | 20 ++++++++++++++++++-- 2 files changed, 31 insertions(+), 2 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 06bbe9eba636..79b3a8cb9a3b 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1913,6 +1913,7 @@ static inline struct folio *virt_to_folio(const void *x) } void __folio_put(struct folio *folio); +void __folio_put_zeroed(struct folio *folio); void split_page(struct page *page, unsigned int order); void folio_copy(struct folio *dst, struct folio *src); @@ -2090,6 +2091,18 @@ static inline void folio_put(struct folio *folio) __folio_put(folio); } +/* Caller must be sole owner to guarantee page is still zero */ +static inline void folio_put_zeroed(struct folio *folio) +{ + if (folio_put_testzero(folio)) + __folio_put_zeroed(folio); +} + +static inline void put_page_zeroed(struct page *page) +{ + folio_put_zeroed(page_folio(page)); +} + /** * folio_put_refs - Reduce the reference count on a folio. * @folio: The folio. diff --git a/mm/swap.c b/mm/swap.c index 5cc44f0de987..ecec780172ad 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -94,13 +94,15 @@ static void page_cache_release(struct folio *folio) lruvec_unlock_irqrestore(lruvec, flags); } -void __folio_put(struct folio *folio) +static void ___folio_put(struct folio *folio, bool zeroed) { + /* zeroed hint ignored for now, no current user */ if (unlikely(folio_is_zone_device(folio))) { free_zone_device_folio(folio); return; } + /* zeroed hint ignored for now, no current user */ if (folio_test_hugetlb(folio)) { free_huge_folio(folio); return; @@ -109,10 +111,24 @@ void __folio_put(struct folio *folio) page_cache_release(folio); folio_unqueue_deferred_split(folio); mem_cgroup_uncharge(folio); - free_frozen_pages(&folio->page, folio_order(folio)); + if (zeroed) + free_frozen_pages_zeroed(&folio->page, folio_order(folio)); + else + free_frozen_pages(&folio->page, folio_order(folio)); +} + +void __folio_put(struct folio *folio) +{ + ___folio_put(folio, false); } EXPORT_SYMBOL(__folio_put); +void __folio_put_zeroed(struct folio *folio) +{ + ___folio_put(folio, true); +} +EXPORT_SYMBOL(__folio_put_zeroed); + typedef void (*move_fn_t)(struct lruvec *lruvec, struct folio *folio); static void lru_add(struct lruvec *lruvec, struct folio *folio) -- MST
