Re: [PATCH v5 24/38] mm/vma: enforce that mm-managed mappings may not set VMA_IO_BIT

On 10/12/26 20:35, Lorenzo Stoakes (ARM) wrote:

On Mon, Oct 12, 2026 at 08:25:54PM +0200, David Hildenbrand (Arm) wrote:

quoted

On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote:

quoted

It makes no sense for a mapping whose contents the kernel does not own to
specify that the range is MMIO.

Prior to this patch, all in-tree drivers which did so have been updated
such that they are marked as not being mm-managed. The check WARNs and
fails the mmap for any out-of-tree driver that still sets VMA_IO_BIT
without a kernel mapping.

No functional change intended for in-tree code.

Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/vma.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/vma.c b/mm/vma.c
index 4f09fb98a2fd..51bdbe08bb02 100644
--- a/mm/vma.c
+++ b/mm/vma.c

@@ -2796,6 +2796,12 @@ static int mmap_validate_vma_flags(const vma_flags_t *flags)
 		return -EINVAL;
 #endif

+	if (!vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT)) {
+		/* mm-managed mappings may not set VMA_IO_BIT. */
+		if (WARN_ON_ONCE(vma_flags_test(flags, VMA_IO_BIT)))
+			return -EINVAL;

We do have a helper (vma_flags_is_mm_managed()) but we're not using it because
of the VM_IO situation ...

That gets removed.

quoted

That's confusing ...

Could we do something like testing vma_flags_is_mm_managed(), but removing the
VM_IO flag from the checked flags?

I mean we do that in a subsequent commit, I think even the next one?

quoted

Then we can add a comment about VM_IO in the future getting remove from
vma_flags_is_mm_managed() ...

Yeah, but to do that the code would now be something like:

	vma_flags_t masked = flags;

	/*
	  * vma_flags_is_mm_managed() tests VMA_IO_BIT, which will be removed
	 * in the next commit.
	 */
	vma_flags_clear(&flags, VMA_IO_BIT);
	if (!vma_flags_is_mm_managed(masked)) {
		...
	}

Is that really worth it?

I don't think there's really too much confusion intra-patch about that is there?

I would do it as above (- the comment, as that is the next patch indeed) and
only remove the vma_flags_clear() in the next patch.

But in any case, you should spell out that this is only temporary in the patch
description :)

-- 
Cheers,

David