Re: [PATCH v3 17/40] mm/vma: add and use vma_[flags]_is_fixed_mapping

This determines whether a VMA cannot be expanded or merged because what
they mapped was determined to be a set size at mmap time.

This typically refers to kernel-owned mappings, however VMA_DONTEXPAND_BIT
is not reliably set alongside VMA_PFNMAP_BIT or VMA_MIXEDMAP_BIT, so we
must explicitly test for this for now.

We also explicitly test for VMA_PFNMAP_BIT as VMA_DONTEXPAND_BIT may not be
set for VMA_PFNMAP_BIT's despite the one implying the other.

Use this predicate in vma_flags_can_merge() and in check_prep_vma() in the
mremap logic testing to see if mremap() can expand the VMA. The criteria
for khugepaged and MADV_COLLAPSE eligibility in
__thp_vma_allowable_orders() are precisely those for mergeability, so use
vma_can_merge() there (with an expanded comment).

This obviates the need for the VM_NO_KHUGEPAGED mask, so remove it.

Hugetlb VMAs remain excluded from khugepaged as hugetlbfs always sets
VMA_DONTEXPAND_BIT.

Also update the userland VMA tests to reflect the change.

No functional change intended.

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

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260917-b4-mmap-prepare-vma-flag-sanify-v3-0-4583d8a23bca@kernel.org?part=17