[PATCH v10 29/37] mm: memfd: skip zeroing for zeroed hugetlb pool pages

["Michael S. Tsirkin" <mst@redhat.com>]

Add bool *zeroed output to alloc_hugetlb_folio_reserve() so
callers can check whether the pool page is known-zero.  memfd's
memfd_alloc_folio() uses this to skip the explicit folio_zero_user()
when the page is already zero.

This avoids redundant zeroing for memfd hugetlb pages that were
pre-allocated into the pool and never mapped to userspace.

Note: HPG_zeroed is currently only set for surplus pages
allocated with __GFP_ZERO (via alloc_surplus_hugetlb_folio),
not for pool pages from alloc_pool_huge_folio. So the
zeroed output from alloc_hugetlb_folio_reserve is typically
false for pool-only reservations. It becomes true when
surplus pages fill the reservation. The addr_hint 0 passed
to folio_zero_user is acceptable for memfd: these pages are
not mapped yet and will get proper dcache handling at mmap
time via the page fault path.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
---
 include/linux/cma.h     |  3 ++-
 include/linux/hugetlb.h |  6 ++++--
 mm/cma.c                |  6 ++++--
 mm/hugetlb.c            | 11 +++++++++--
 mm/hugetlb_cma.c        |  4 ++--
 mm/memfd.c              | 14 ++++++++------
 6 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/include/linux/cma.h b/include/linux/cma.h
index 8555d38a97b1..dee88909cf5d 100644
--- a/include/linux/cma.h
+++ b/include/linux/cma.h
@@ -53,7 +53,8 @@ extern bool cma_release(struct cma *cma, const struct page *pages, unsigned long
 
 struct page *cma_alloc_frozen(struct cma *cma, unsigned long count,
 		unsigned int align, bool no_warn);
-struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order);
+struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order,
+				       gfp_t caller_gfp);
 bool cma_release_frozen(struct cma *cma, const struct page *pages,
 		unsigned long count);
 
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 06d033a57a61..7eb529eabe99 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -708,7 +708,8 @@ struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
 				nodemask_t *nmask, gfp_t gfp_mask,
 				bool allow_alloc_fallback);
 struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
-					  nodemask_t *nmask, gfp_t gfp_mask);
+					  nodemask_t *nmask, gfp_t gfp_mask,
+					  bool *zeroed);
 
 int hugetlb_add_to_page_cache(struct folio *folio, struct address_space *mapping,
 			pgoff_t idx);
@@ -1128,7 +1129,8 @@ static inline void wait_for_freed_hugetlb_folios(void)
 
 static inline struct folio *
 alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
-			    nodemask_t *nmask, gfp_t gfp_mask)
+			    nodemask_t *nmask, gfp_t gfp_mask,
+			    bool *zeroed)
 {
 	return NULL;
 }
diff --git a/mm/cma.c b/mm/cma.c
index c7ca567f4c5c..27971f6264ab 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -924,9 +924,11 @@ struct page *cma_alloc_frozen(struct cma *cma, unsigned long count,
 	return __cma_alloc_frozen(cma, count, align, gfp);
 }
 
-struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order)
+struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order,
+				       gfp_t caller_gfp)
 {
-	gfp_t gfp = GFP_KERNEL | __GFP_COMP | __GFP_NOWARN;
+	gfp_t gfp = GFP_KERNEL | __GFP_COMP | __GFP_NOWARN |
+		    (caller_gfp & __GFP_ZERO);
 
 	return __cma_alloc_frozen(cma, 1 << order, order, gfp);
 }
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed00db703911..a087e915783f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2196,7 +2196,7 @@ struct folio *alloc_buddy_hugetlb_folio_with_mpol(struct hstate *h,
 }
 
 struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
-		nodemask_t *nmask, gfp_t gfp_mask)
+		nodemask_t *nmask, gfp_t gfp_mask, bool *zeroed)
 {
 	struct folio *folio;
 
@@ -2212,6 +2212,12 @@ struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
 		h->resv_huge_pages--;
 
 	spin_unlock_irq(&hugetlb_lock);
+
+	if (zeroed && folio) {
+		*zeroed = folio_test_hugetlb_zeroed(folio);
+		folio_clear_hugetlb_zeroed(folio);
+	}
+
 	return folio;
 }
 
@@ -2296,7 +2302,8 @@ static int gather_surplus_pages(struct hstate *h, long delta)
 		 * It is okay to use NUMA_NO_NODE because we use numa_mem_id()
 		 * down the road to pick the current node if that is the case.
 		 */
-		folio = alloc_surplus_hugetlb_folio(h, htlb_alloc_mask(h),
+		folio = alloc_surplus_hugetlb_folio(h,
+						    htlb_alloc_mask(h),
 						    NUMA_NO_NODE, &alloc_nodemask,
 						    USER_ADDR_NONE);
 		if (!folio) {
diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
index 7693ccefd0c6..c9266b25be3d 100644
--- a/mm/hugetlb_cma.c
+++ b/mm/hugetlb_cma.c
@@ -35,14 +35,14 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
 		return NULL;
 
 	if (hugetlb_cma[nid])
-		page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
+		page = cma_alloc_frozen_compound(hugetlb_cma[nid], order, gfp_mask);
 
 	if (!page && !(gfp_mask & __GFP_THISNODE)) {
 		for_each_node_mask(node, *nodemask) {
 			if (node == nid || !hugetlb_cma[node])
 				continue;
 
-			page = cma_alloc_frozen_compound(hugetlb_cma[node], order);
+			page = cma_alloc_frozen_compound(hugetlb_cma[node], order, gfp_mask);
 			if (page)
 				break;
 		}
diff --git a/mm/memfd.c b/mm/memfd.c
index abe13b291ddc..a99617a62e33 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -69,6 +69,7 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 #ifdef CONFIG_HUGETLB_PAGE
 	struct folio *folio;
 	gfp_t gfp_mask;
+	bool zeroed;
 
 	if (is_file_hugepages(memfd)) {
 		/*
@@ -93,17 +94,18 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 		folio = alloc_hugetlb_folio_reserve(h,
 						    numa_node_id(),
 						    NULL,
-						    gfp_mask);
+						    gfp_mask,
+						    &zeroed);
 		if (folio) {
 			u32 hash;
 
 			/*
-			 * Zero the folio to prevent information leaks to userspace.
-			 * Use folio_zero_user() which is optimized for huge/gigantic
-			 * pages. Pass 0 as addr_hint since this is not a faulting path
-			 *  and we don't have a user virtual address yet.
+			 * Zero the folio to prevent information leaks to
+			 * userspace.  Skip if the pool page is known-zero
+			 * (HPG_zeroed set during pool pre-allocation).
 			 */
-			folio_zero_user(folio, 0);
+			if (!zeroed)
+				folio_zero_user(folio, 0);
 
 			/*
 			 * Mark the folio uptodate before adding to page cache,
-- 
MST