linux-kernel - [PATCH v2 2/4] mm/shmem: Support vm_uffd

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Message-ID: <20250627154655.2085903-3-peterx@redhat.com>
Date: Fri, 27 Jun 2025 11:46:53 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Mike Rapoport <rppt@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Houghton <jthoughton@...gle.com>,
	peterx@...hat.com,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Michal Hocko <mhocko@...e.com>,
	David Hildenbrand <david@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Oscar Salvador <osalvador@...e.de>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>
Subject: [PATCH v2 2/4] mm/shmem: Support vm_uffd_ops API

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

Due to the tailored uffd_copy() API, shmem is extremely easy to support it
by reusing the existing mfill function.

It only needs a separate uffd_get_folio() definition but that's oneliner.

Cc: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/shmem.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2b19965d27df..9a8b8dd4709b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3151,6 +3151,13 @@ static inline struct inode *shmem_get_inode(struct mnt_idmap *idmap,
 #endif /* CONFIG_TMPFS_QUOTA */
 
 #ifdef CONFIG_USERFAULTFD
+
+static int shmem_uffd_get_folio(struct inode *inode, pgoff_t pgoff,
+				struct folio **folio)
+{
+	return shmem_get_folio(inode, pgoff, 0, folio, SGP_NOALLOC);
+}
+
 int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
 			   struct vm_area_struct *dst_vma,
 			   unsigned long dst_addr,
@@ -5194,6 +5201,19 @@ static int shmem_error_remove_folio(struct address_space *mapping,
 	return 0;
 }
 
+#ifdef CONFIG_USERFAULTFD
+static const vm_uffd_ops shmem_uffd_ops = {
+	.uffd_features	= 	__VM_UFFD_FLAGS,
+	.uffd_ioctls	= 	BIT(_UFFDIO_COPY) |
+				BIT(_UFFDIO_ZEROPAGE) |
+				BIT(_UFFDIO_WRITEPROTECT) |
+				BIT(_UFFDIO_CONTINUE) |
+				BIT(_UFFDIO_POISON),
+	.uffd_get_folio	=	shmem_uffd_get_folio,
+	.uffd_copy	=	shmem_mfill_atomic_pte,
+};
+#endif
+
 static const struct address_space_operations shmem_aops = {
 	.dirty_folio	= noop_dirty_folio,
 #ifdef CONFIG_TMPFS
@@ -5296,6 +5316,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.userfaultfd_ops = &shmem_uffd_ops,
+#endif
 };
 
 static const struct vm_operations_struct shmem_anon_vm_ops = {
@@ -5305,6 +5328,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.userfaultfd_ops = &shmem_uffd_ops,
+#endif
 };
 
 int shmem_init_fs_context(struct fs_context *fc)
-- 
2.49.0