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

[Zi Yan]

On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) wrote:
> 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@xxxxxxxxxx>
> ---
>  include/linux/mm.h              | 39 +++++++++++++++++++++++++++++++++++----
>  mm/huge_memory.c                | 11 +++++++----
>  mm/mremap.c                     |  5 ++---
>  tools/testing/vma/include/dup.h | 16 +++++++++++++++-
>  4 files changed, 59 insertions(+), 12 deletions(-)
>
<snip>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4cd917f77f3f..4d0acd9a1099 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -212,11 +212,14 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  		return in_pf ? orders : 0;
>  
>  	/*
> -	 * khugepaged special VMA and hugetlb VMA.
> -	 * Must be checked after dax since some dax mappings may have
> -	 * VM_MIXEDMAP set.
> +	 * khugepaged moves data from VMAs once collapsed, after they have been
> +	 * faulted in, relying on refaulting for file-backed memory.
> +	 *
> +	 * Kernel-owned mappings cannot be reliably reconstructed from page
> +	 * faults, and fixed mappings (including hugetlb) may not be marked as
> +	 * kernel-owned - precisely the mappings which cannot be merged.
>  	 */
> -	if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))
> +	if (!in_pf && !smaps && !vma_can_merge(vma))

I wonder if a function alias would improve the code readability.
Basically,

#define vma_no_khugepaged vma_can_merge

or just make vma_no_khugepaged static inline. And move the comment to
the function.

Regardless, this patch makes sense to me.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


-- 
Best Regards,
Yan, Zi