Re: [PATCH 10/13] mm/vma: convert miscellaneous uses of VMA flags in core mm

On Thu, Jul 02, 2026 at 09:12:33PM +0800, Lance Yang wrote:
>
> On Mon, Jun 29, 2026 at 08:25:33PM +0100, Lorenzo Stoakes wrote:
> >Update various uses of legacy flags in vma.c and mmap.c to the new
> >vma_flags_t type, updating comments alongside them to be consistent.
> >
> >Also update __install_special_mapping() to rearrange things slightly to
> >accommodate the changes.
> >
> >Signed-off-by: Lorenzo Stoakes <[email protected]>
> >---
> [...]
> >diff --git a/mm/vma.c b/mm/vma.c
> >index b81c05e67a61..ab2ef0f04420 100644
> >--- a/mm/vma.c
> >+++ b/mm/vma.c
> >@@ -3417,23 +3417,27 @@ struct vm_area_struct *__install_special_mapping(
> > 	vm_flags_t vm_flags, void *priv,
> > 	const struct vm_operations_struct *ops)
> > {
> >-	int ret;
> >+	vma_flags_t vma_flags = legacy_to_vma_flags(vm_flags);
> > 	struct vm_area_struct *vma;
> >+	int ret;
> >
> > 	vma = vm_area_alloc(mm);
> >-	if (unlikely(vma == NULL))
> >+	if (unlikely(!vma))
> > 		return ERR_PTR(-ENOMEM);
> >
> >-	vma_set_range(vma, addr, addr + len, 0);
> >-	vm_flags |= vma_flags_to_legacy(mm->def_vma_flags) | VM_DONTEXPAND;
> >+	vma_flags_set_mask(&vma_flags, mm->def_vma_flags);
> >+	vma_flags_set(&vma_flags, VMA_DONTEXPAND_BIT);
> > 	if (pgtable_supports_soft_dirty())
> >-		vm_flags |= VM_SOFTDIRTY;
> >-	vm_flags_init(vma, vm_flags & ~VM_LOCKED_MASK);
> >+		vma_flags_set(&vma_flags, VMA_SOFTDIRTY_BIT);
> >+	vma_flags_clear_mask(&vma_flags, VMA_LOCKED_MASK);
> >+	vma->flags = vma_flags;
>
> Maybe worth a vma_flags_init() helper here to mirror vm_flags_init()?
> With this open-coded, we lose the soft-dirty WARN_ON_ONCE sanity check.
>
> Might be nicer to keep that check in one place ;)

I really hate all the VMA flag accessors, they conflate things horribly - we
should be explicitly taking VMA write locks when we need to (and often killable
ones actually) not assuming that a VMA flags accessor does (they should at most
assert).

This case is even more terribly egregious - you are setting flags at an
arbitrary time, why are we asserting something about softdirty?

You may update them as part of initialisation, maybe not. It's far from a
guarantee and feels like a lazy place to put it.

BUT obviously it's an oversight not to open code that here, so I'll update the
patch to do that!

I want VMA flags to be a clean stateless thing, other than the flags
themselves. Implicit, unrelated, asserts or lock acquisitions in general should
be done separately IMO.

>
> [...]

Thanks, Lorenzo