linux-kernel - [PATCH v4 3/4] mm/hugetlb: Support vm_uffd

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Message-ID: <20251014231501.2301398-4-peterx@redhat.com>
Date: Tue, 14 Oct 2025 19:15:00 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc: Mike Rapoport <rppt@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Houghton <jthoughton@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	peterx@...hat.com,
	Oscar Salvador <osalvador@...e.de>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH v4 3/4] mm/hugetlb: Support vm_uffd_ops API

Add support for the new vm_uffd_ops API for hugetlb.  Note that this only
introduces the support, the API is not yet used by core mm.

Due to legacy reasons, it's still not trivial to move hugetlb completely to
the API.  But it will still use supported_ioctls properly on the API with
all the implementations still hard coded in mm/.

Cc: Muchun Song <muchun.song@...ux.dev>
Cc: Oscar Salvador <osalvador@...e.de>
Acked-by: Mike Rapoport <rppt@...nel.org>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/hugetlb.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 657b9facba280..ad98876d338e5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5503,6 +5503,21 @@ static vm_fault_t hugetlb_vm_op_fault(struct vm_fault *vmf)
 	return 0;
 }
 
+#ifdef CONFIG_USERFAULTFD
+static const struct vm_uffd_ops hugetlb_uffd_ops = {
+	/* _UFFDIO_ZEROPAGE not supported */
+	.supported_ioctls	=	BIT(_UFFDIO_COPY) |
+					BIT(_UFFDIO_WRITEPROTECT) |
+					BIT(_UFFDIO_CONTINUE) |
+					BIT(_UFFDIO_POISON),
+	/*
+	 * Hugetlbfs still has its own hard-coded handler in userfaultfd,
+	 * due to limitations similar to vm_operations_struct.fault().
+	 * TODO: generalize it to use the API functions.
+	 */
+};
+#endif
+
 /*
  * When a new function is introduced to vm_operations_struct and added
  * to hugetlb_vm_ops, please consider adding the function to shm_vm_ops.
@@ -5516,6 +5531,9 @@ const struct vm_operations_struct hugetlb_vm_ops = {
 	.close = hugetlb_vm_op_close,
 	.may_split = hugetlb_vm_op_split,
 	.pagesize = hugetlb_vm_op_pagesize,
+#ifdef CONFIG_USERFAULTFD
+	.userfaultfd_ops = &hugetlb_uffd_ops,
+#endif
 };
 
 static pte_t make_huge_pte(struct vm_area_struct *vma, struct folio *folio,
-- 
2.50.1