Subject: Re: [PATCH v3 22/23] mm/vma: convert vma_modify_flags[_uffd]() to use vma_flags_t

From: "Vlastimil Babka (SUSE)" <[email-masked]>
Date: Fri, 20 Mar 2026 11:39:58 +0100

On 3/18/26 16:50, Lorenzo Stoakes (Oracle) wrote:
> Update the vma_modify_flags() and vma_modify_flags_uffd() functions to
> accept a vma_flags_t parameter rather than a vm_flags_t one, and propagate
> the changes as needed to implement this change.
> 
> Finally, update the VMA tests to reflect this.
> 
> Signed-off-by: Lorenzo Stoakes (Oracle) <[email-masked]>

> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -415,13 +415,14 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
>   * @vma - vma containing range to be mlock()ed or munlock()ed
>   * @start - start address in @vma of the range
>   * @end - end of range in @vma
> - * @newflags - the new set of flags for @vma.
> + * @new_vma_flags - the new set of flags for @vma.
>   *
>   * Called for mlock(), mlock2() and mlockall(), to set @vma VM_LOCKED;
>   * called for munlock() and munlockall(), to clear VM_LOCKED from @vma.
>   */
>  static void mlock_vma_pages_range(struct vm_area_struct *vma,
> -	unsigned long start, unsigned long end, vm_flags_t newflags)
> +	unsigned long start, unsigned long end,
> +	vma_flags_t *new_vma_flags)
>  {
>  	static const struct mm_walk_ops mlock_walk_ops = {
>  		.pmd_entry = mlock_pte_range,
> @@ -439,18 +440,18 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
>  	 * combination should not be visible to other mmap_lock users;
>  	 * but WRITE_ONCE so rmap walkers must see VM_IO if VM_LOCKED.
>  	 */
> -	if (newflags & VM_LOCKED)
> -		newflags |= VM_IO;
> +	if (vma_flags_test(new_vma_flags, VMA_LOCKED_BIT))
> +		vma_flags_set(new_vma_flags, VMA_IO_BIT);
>  	vma_start_write(vma);
> -	vm_flags_reset_once(vma, newflags);
> +	WRITE_ONCE(vma->flags, *new_vma_flags);

It's not clear to me, how is switching from vm_flags_t to vma_flags_t
allowing us to simply do WRITE_ONCE() instead of the full logic of
vm_flags_reset_once()? Won't it fail to compile once once flags are more
than single word? Or worse, will compile but silently allow tearing?

>  
>  	lru_add_drain();
>  	walk_page_range(vma->vm_mm, start, end, &mlock_walk_ops, NULL);
>  	lru_add_drain();
>  
> -	if (newflags & VM_IO) {
> -		newflags &= ~VM_IO;
> -		vm_flags_reset_once(vma, newflags);
> +	if (vma_flags_test(new_vma_flags, VMA_IO_BIT)) {
> +		vma_flags_clear(new_vma_flags, VMA_IO_BIT);
> +		WRITE_ONCE(vma->flags, *new_vma_flags);

Ditto.

>  	}
>  }
>  
> @@ -467,20 +468,22 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	       struct vm_area_struct **prev, unsigned long start,
>  	       unsigned long end, vm_flags_t newflags)
>  {
> +	vma_flags_t new_vma_flags = legacy_to_vma_flags(newflags);
> +	const vma_flags_t old_vma_flags = vma->flags;
>  	struct mm_struct *mm = vma->vm_mm;
>  	int nr_pages;
>  	int ret = 0;
> -	vm_flags_t oldflags = vma->vm_flags;
>  
> -	if (newflags == oldflags || vma_is_secretmem(vma) ||
> -	    !vma_supports_mlock(vma))
> +	if (vma_flags_same_pair(&old_vma_flags, &new_vma_flags) ||
> +	    vma_is_secretmem(vma) || !vma_supports_mlock(vma)) {
>  		/*
>  		 * Don't set VM_LOCKED or VM_LOCKONFAULT and don't count.
>  		 * For secretmem, don't allow the memory to be unlocked.
>  		 */
>  		goto out;
> +	}
>  
> -	vma = vma_modify_flags(vmi, *prev, vma, start, end, &newflags);
> +	vma = vma_modify_flags(vmi, *prev, vma, start, end, &new_vma_flags);
>  	if (IS_ERR(vma)) {
>  		ret = PTR_ERR(vma);
>  		goto out;
> @@ -490,9 +493,9 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 * Keep track of amount of locked VM.
>  	 */
>  	nr_pages = (end - start) >> PAGE_SHIFT;
> -	if (!(newflags & VM_LOCKED))
> +	if (!vma_flags_test(&new_vma_flags, VMA_LOCKED_BIT))
>  		nr_pages = -nr_pages;
> -	else if (oldflags & VM_LOCKED)
> +	else if (vma_flags_test(&old_vma_flags, VMA_LOCKED_BIT))
>  		nr_pages = 0;
>  	mm->locked_vm += nr_pages;
>  
> @@ -501,12 +504,13 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 * It's okay if try_to_unmap_one unmaps a page just after we
>  	 * set VM_LOCKED, populate_vma_page_range will bring it back.
>  	 */
> -	if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
> +	if (vma_flags_test(&new_vma_flags, VMA_LOCKED_BIT) &&
> +	    vma_flags_test(&old_vma_flags, VMA_LOCKED_BIT)) {
>  		/* No work to do, and mlocking twice would be wrong */
>  		vma_start_write(vma);
> -		vm_flags_reset(vma, newflags);
> +		vma->flags = new_vma_flags;

This also does lot less than vm_flags_reset()?

>  	} else {
> -		mlock_vma_pages_range(vma, start, end, newflags);
> +		mlock_vma_pages_range(vma, start, end, &new_vma_flags);
>  	}
>  out:
>  	*prev = vma;
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index eaa724b99908..2b8a85689ab7 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -756,13 +756,11 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  		vma_flags_clear(&new_vma_flags, VMA_ACCOUNT_BIT);
>  	}
>  
> -	newflags = vma_flags_to_legacy(new_vma_flags);
> -	vma = vma_modify_flags(vmi, *pprev, vma, start, end, &newflags);
> +	vma = vma_modify_flags(vmi, *pprev, vma, start, end, &new_vma_flags);
>  	if (IS_ERR(vma)) {
>  		error = PTR_ERR(vma);
>  		goto fail;
>  	}
> -	new_vma_flags = legacy_to_vma_flags(newflags);
>  
>  	*pprev = vma;
>  
> @@ -771,7 +769,7 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	 * held in write mode.
>  	 */
>  	vma_start_write(vma);
> -	vm_flags_reset_once(vma, newflags);
> +	WRITE_ONCE(vma->flags, new_vma_flags);

Ditto.

>  	if (vma_wants_manual_pte_write_upgrade(vma))
>  		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
>  	vma_set_page_prot(vma);
> @@ -796,6 +794,7 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	}
>  
>  	vm_stat_account(mm, vma_flags_to_legacy(old_vma_flags), -nrpages);
> +	newflags = vma_flags_to_legacy(new_vma_flags);
>  	vm_stat_account(mm, newflags, nrpages);
>  	perf_event_mmap(vma);
>  	return 0;
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 316b5e1dec78..603df53ad267 100644