linux-kernel - [PATCH v3 3/4] mm/hugetlb: Support vm_uffd

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Message-ID: <20250926211650.525109-4-peterx@redhat.com>
Date: Fri, 26 Sep 2025 17:16:49 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Axel Rasmussen <axelrasmussen@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	James Houghton <jthoughton@...gle.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>,
	Mike Rapoport <rppt@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	peterx@...hat.com,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Michal Hocko <mhocko@...e.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>,
	Hugh Dickins <hughd@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH v3 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 (like shmem).  But it will still use uffd_features and uffd_ioctls
properly on the API because that's pretty general.

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 | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index cb5c4e79e0b8f..b6eb92828ee15 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5492,6 +5492,22 @@ 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 = {
+	.uffd_features	=	__VM_UFFD_FLAGS,
+	/* _UFFDIO_ZEROPAGE not supported */
+	.uffd_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.
@@ -5505,6 +5521,9 @@ 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