Re: [PATCH v3 2/8] mm: add atomic VMA flags and set VM_MAYBE_GUARD as such

[Vlastimil Babka <vbabka@suse.cz>]

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;