linux-kernel - [PATCH 3/5] mm: introduce VM_FAULT_UFFD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251123102707.559422-4-rppt@kernel.org>
Date: Sun, 23 Nov 2025 12:27:05 +0200
From: Mike Rapoport <rppt@...nel.org>
To: linux-mm@...ck.org
Cc: Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Houghton <jthoughton@...gle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Michal Hocko <mhocko@...e.com>,
	Mike Rapoport <rppt@...nel.org>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Xu <peterx@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	"David Hildenbrand (Red Hat)" <david@...nel.org>
Subject: [PATCH 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>

When a VMA is registered with userfaulfd in minor mode, its ->fault()
method should check if a folio exists in the page cache and if yes
->fault() should call handle_userfault(VM_UFFD_MISSING).

Instead of calling handle_userfault() directly from a specific ->fault()
implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will
notify the core page fault handler that it should call
handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.

Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the
new VM_FAULT_UFFD_MINOR there instead.

Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
---
 include/linux/mm_types.h | 3 +++
 mm/memory.c              | 2 ++
 mm/shmem.c               | 2 +-
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 90e5790c318f..eb135369940f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t;
  *				fsync() to complete (for synchronous page faults
  *				in DAX)
  * @VM_FAULT_COMPLETED:		->fault completed, meanwhile mmap lock released
+ * @VM_FAULT_UFFD_MINOR:	->fault did not modify page tables and needs
+ *				handle_userfault(VM_UFFD_MINOR) to complete
  * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
  *
  */
@@ -1540,6 +1542,7 @@ enum vm_fault_reason {
 	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
 	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
 	VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
+	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x008000,
 	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
 };
 
diff --git a/mm/memory.c b/mm/memory.c
index b59ae7ce42eb..94acbac8cefb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 	}
 
 	ret = vma->vm_ops->fault(vmf);
+	if (unlikely(ret & VM_FAULT_UFFD_MINOR))
+		return handle_userfault(vmf, VM_UFFD_MINOR);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
 			    VM_FAULT_DONE_COW)))
 		return ret;
diff --git a/mm/shmem.c b/mm/shmem.c
index aaa21bb60f51..6dcb73b52bcc 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	if (folio && vma && userfaultfd_minor(vma)) {
 		if (!xa_is_value(folio))
 			folio_put(folio);
-		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
+		*fault_type = VM_FAULT_UFFD_MINOR;
 		return 0;
 	}
 
-- 
2.50.1