On 11/7/25 17:11, Lorenzo Stoakes wrote:
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@suse.de Reviewed-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Lorenzo Stoakes lorenzo.stoakes@oracle.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);+}
Hm clang is unhappy here.
./include/linux/mm.h:932:1: error: non-void function does not return a value in all control paths [-Werror,-Wreturn-type] 932 | } | ^ 1 error generated.
I don't have CONFIG_WERROR enabled though, so not sure why it's not just a warning, as the function is unused until patch 5/8 which adds a "return false" here. So it's just a potential bisection annoyance with clang.
Andrew could you move that hunk from to this patch? Thanks.
static inline void vma_set_anonymous(struct vm_area_struct *vma) { vma->vm_ops = NULL;