Re: [PATCH v2 2/5] mm: add atomic VMA flags, use VM_MAYBE_GUARD as such

[Pedro Falcato <pfalcato@suse.de>]

On Thu, Nov 06, 2025 at 10:46:13AM +0000, 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, and then set it under the mmap
> read flag upon guard region installation.
> 
> The places where this flag is used currently and matter are:
> 
> * VMA merge - performed under mmap/VMA write lock, therefore excluding
>   racing writes.
> 
> * /proc/$pid/smaps - can race the write, however this isn't meaningful as
>   the flag write is performed at the point of the guard region being
>   established, and thus an smaps reader can't reasonably expect to avoid
>   races. Due to atomicity, a reader will observe either the flag being set
>   or not. Therefore consistency will be maintained.
> 
> In all other cases the flag being set is irrelevant and atomicity
> guarantees other flags will be read correctly.

Probably important to write down that the only reason why this doesn't make
KCSAN have a small stroke is that we are only changing one bit. i.e we can
only have one bit of atomic flags before annotating every reader.

(Source: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/kcsan/permissive.h#n51)

> We additionally update madvise_guard_install() to ensure that
> anon_vma_prepare() is set for anonymous VMAs to maintain consistency with
> the assumption that any anonymous VMA with page tables will have an
> anon_vma set, and any with an anon_vma unset will not have page tables
> established.

Isn't that what we already had? Or do you mean "*only* set for anonymous VMAs"?

> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

With the nits below and above addressed:
Reviewed-by: Pedro Falcato <pfalcato@suse.de>

> ---
>  include/linux/mm.h | 23 +++++++++++++++++++++++
>  mm/madvise.c       | 22 ++++++++++++++--------
>  2 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2a5516bff75a..2ea65c646212 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 +863,26 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
>  	__vm_flags_mod(vma, set, clear);
>  }
>  
> +/*
> + * 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)
> +{
> +	const vm_flags_t mask = BIT(bit);
> +
> +	/* mmap read lock/VMA read lock must be held. */
> +	if (!rwsem_is_locked(&vma->vm_mm->mmap_lock))
> +		vma_assert_locked(vma);
> +
> +	/* Only specific flags are permitted */
> +	if (WARN_ON_ONCE(!(mask & VM_ATOMIC_SET_ALLOWED)))
> +		return;

VM_WARN_ON_ONCE?

-- 
Pedro