linux-kernel - [PATCH v3 2/8] mm: add atomic VMA flags and set VM_MAYBE

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Message-ID: <cda9d4c073d773ef6c2cf2939d66cf80544cff40.1762531708.git.lorenzo.stoakes@oracle.com>
Date: Fri,  7 Nov 2025 16:11:47 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, Andrei Vagin <avagin@...il.com>
Subject: [PATCH v3 2/8] mm: add atomic VMA flags and set VM_MAYBE_GUARD as such

This patch adds the ability to atomically set VMA flags with only the mmap
read/VMA read lock held.

As this could be hugely problematic for VMA flags in general given that all
other accesses are non-atomic and serialised by the mmap/VMA locks, we
implement this with a strict allow-list - that is, only designated flags
are allowed to do this.

We make VM_MAYBE_GUARD one of these flags.

Reviewed-by: Pedro Falcato <pfalcato@...e.de>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 include/linux/mm.h | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2a5516bff75a..699566c21ff7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -518,6 +518,9 @@ extern unsigned int kobjsize(const void *objp);
 /* This mask represents all the VMA flag bits used by mlock */
 #define VM_LOCKED_MASK	(VM_LOCKED | VM_LOCKONFAULT)
 
+/* These flags can be updated atomically via VMA/mmap read lock. */
+#define VM_ATOMIC_SET_ALLOWED VM_MAYBE_GUARD
+
 /* Arch-specific flags to clear when updating VM flags on protection change */
 #ifndef VM_ARCH_CLEAR
 # define VM_ARCH_CLEAR	VM_NONE
@@ -860,6 +863,45 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
 	__vm_flags_mod(vma, set, clear);
 }
 
+static inline bool __vma_flag_atomic_valid(struct vm_area_struct *vma,
+				       int bit)
+{
+	const vm_flags_t mask = BIT(bit);
+
+	/* Only specific flags are permitted */
+	if (WARN_ON_ONCE(!(mask & VM_ATOMIC_SET_ALLOWED)))
+		return false;
+
+	return true;
+}
+
+/*
+ * Set VMA flag atomically. Requires only VMA/mmap read lock. Only specific
+ * valid flags are allowed to do this.
+ */
+static inline void vma_flag_set_atomic(struct vm_area_struct *vma, int bit)
+{
+	/* mmap read lock/VMA read lock must be held. */
+	if (!rwsem_is_locked(&vma->vm_mm->mmap_lock))
+		vma_assert_locked(vma);
+
+	if (__vma_flag_atomic_valid(vma, bit))
+		set_bit(bit, &vma->__vm_flags);
+}
+
+/*
+ * Test for VMA flag atomically. Requires no locks. Only specific valid flags
+ * are allowed to do this.
+ *
+ * This is necessarily racey, so callers must ensure that serialisation is
+ * achieved through some other means, or that races are permissible.
+ */
+static inline bool vma_flag_test_atomic(struct vm_area_struct *vma, int bit)
+{
+	if (__vma_flag_atomic_valid(vma, bit))
+		return test_bit(bit, &vma->__vm_flags);
+}
+
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
 {
 	vma->vm_ops = NULL;
-- 
2.51.0