Re: [PATCH v3 02/40] mm/vma: predicate setting mmap_prepare VMA fields on new vma alloc

On Wed, Sep 23, 2026 at 08:32:43AM -0700, Suren Baghdasaryan wrote:

On Thu, Sep 17, 2026 at 9:24 AM Lorenzo Stoakes (ARM) [off-list ref] wrote:

quoted

It only makes sense to manipulate VMA fields if we allocated a new VMA,
rather than merged it.

VMA merging does not compare vm_ops or vm_private_data, so a merged VMA
keeps its own, which is also what the legacy f_op->mmap path does since it
never touches an existing VMA. Previously set_vma_user_defined_fields()
overwrote the merged VMA's fields with those set for the new mapping. In
practice these are the same values, with rare exceptions such as shmem
selecting vm_ops based on whether the file has been unlinked, so no
user-visible change is expected.

Make this dependency explicit, and additionally constify have_mmap_prepare
while we're here.

The fact that we might be overriding attributes of an existing VMA
that we merged with is technically a bug even if we never hit it,
right? If so, should we have:

Fixes: c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback")

It's not a bug, it is an in-built assumption that the state used to assess
mergeability implies the same properties.

And if it was, it'd need fixing a different way (check the field for instance)
and would apply to the legacy mmap hook also.

This change is needed for the series though.

quoted

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks!

quoted

---
 mm/vma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index fa784f069da4..4b74b48c29b7 100644
--- a/mm/vma.c
+++ b/mm/vma.c

@@ -2894,7 +2894,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 {
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = NULL;
-       bool have_mmap_prepare = file && file->f_op->mmap_prepare;
+       const bool have_mmap_prepare = file && file->f_op->mmap_prepare;
        VMA_ITERATOR(vmi, mm, addr);
        const pgoff_t anon_pgoff = addr >> PAGE_SHIFT;
        MMAP_STATE(map, mm, &vmi, addr, len, pgoff, anon_pgoff, vma_flags, file);

@@ -2937,7 +2937,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
                allocated_new = true;
        }

-       if (have_mmap_prepare && !map_is_anon(&map))
+       if (have_mmap_prepare && allocated_new && !map_is_anon(&map))
                set_vma_user_defined_fields(vma, &map);

        __mmap_complete(&map, vma);

--

2.55.0

--
Cheers, Lorenzo