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

On Thu Jul 2, 2026 at 11:46 AM EDT, Lorenzo Stoakes wrote:
> 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!

What do you want to open code here? softdirty WARN_ON_ONCE()?

vma_flags gets VMA_SOFTDIRTY_BIT just above vma->flags, why do we need a
check after that?

BTW, if you think the check is needed, patch 9 will need to be updated,
since the same pattern appears in create_init_stack_vma().

>
> 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.
>

Anyway,

Reviewed-by: Zi Yan <[email protected]>

-- 
Best Regards,
Yan, Zi