Re: [PATCH] mm: shmem: fix incorrect vm_flags usage when checking allowable orders

[Lorenzo Stoakes (ARM)]

On Mon, Aug 17, 2026 at 03:16:43PM +0800, Baolin Wang wrote:
> Lance reported that when MADV_HUGEPAGE is set on a tmpfs file mounted with
> huge=advise option, khugepaged fails the allowable order check and does not
> scan the tmpfs file for collapse.

Ugh.

But really I think this isn't quite accurate - what you mean to say I think is
that when _nothing else_ causes the mm to be considered for khugepaged collapse,
an MADV_HUGEPAGE-advised tmpfs VMA alone does not cause scanning to commence.

>
> After commit 6beeab870e70 ("mm: shmem: move shmem_huge_global_enabled() into
> shmem_allowable_huge_orders()"), the shmem/tmpfs allowable order check reads
> vma->vm_flags directly.  However, when MADV_HUGEPAGE is handled,
> khugepaged_enter_vma() is called before the VMA's vm_flags have been updated,
> so the check uses stale flags and incorrectly rejects the VMA for collapse.
> As a result, khugepaged does not collapse the tmpfs file into PMD order in time.

Could we at least refer to the non-deprecated field in the commit message?
i.e. vma->flags.

Probably worth mentioning VMA_HUGEPAGE_BIT also.

>
> Fix this by passing vm_flags as a parameter to shmem_allowable_huge_orders()
> instead of reading it from the vm_area_struct.

Ugh this is so disgusting.

I understand this is a fix for a bug to be backported but couldn't we just
achieve the same without having to add a deprecated field to be passed around?

As you say the khugepaged_enter_vma() isn't really so helpful in
hugepage_madvise().

But you could add this to the bottom of madvise_update_vma():

	if (vma_flags_test(&new_vma_flags, VMA_HUGEPAGE_BIT))
		khugepaged_enter_vma(vma, new_flags);

I don't think this is really egregious on this code path and could go some way
towards us eliminating the silly thing of passing around flags-to-be-set.

And yeah you'd endure a little backport pain on switching that new_vma_flags
vma_flags_t test to a new_flags vm_flags_t test but it's not so bad :)

I will be doing a series soon that improves the allowable orders vomit-a-thon
somewhat in tip :)

>
> Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260815181632.21453-1-lance.yang@xxxxxxxxx/
> Fixes: 6beeab870e70 ("mm: shmem: move shmem_huge_global_enabled() into shmem_allowable_huge_orders()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/shmem_fs.h | 8 ++++----
>  mm/huge_memory.c         | 2 +-
>  mm/shmem.c               | 9 +++++----
>  3 files changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 5663dff53186..321017e0fd63 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -127,13 +127,13 @@ int shmem_unuse(unsigned int type);
>
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SHMEM)
>  unsigned long shmem_allowable_huge_orders(struct inode *inode,
> -				struct vm_area_struct *vma, pgoff_t index,
> -				loff_t write_end, bool shmem_huge_force);
> +		struct vm_area_struct *vma, vm_flags_t vm_flags,
> +		pgoff_t index, loff_t write_end, bool shmem_huge_force);
>  bool shmem_hpage_pmd_enabled(void);
>  #else
>  static inline unsigned long shmem_allowable_huge_orders(struct inode *inode,
> -				struct vm_area_struct *vma, pgoff_t index,
> -				loff_t write_end, bool shmem_huge_force)
> +		struct vm_area_struct *vma, vm_flags_t vm_flags, pgoff_t index,
> +		loff_t write_end, bool shmem_huge_force)
>  {
>  	return 0;
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ced400f72d43..70f57d700739 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -187,7 +187,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	 */
>  	if (!in_pf && shmem_file(vma->vm_file))
>  		return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
> -						   vma, vma_start_pgoff(vma), 0,
> +						   vma, vm_flags, vma_start_pgoff(vma), 0,
>  						   forced_collapse);
>
>  	if (!vma_is_anonymous(vma)) {
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 599665a3d6e7..6f1ad5456aca 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1834,12 +1834,11 @@ bool shmem_hpage_pmd_enabled(void)
>  }
>
>  unsigned long shmem_allowable_huge_orders(struct inode *inode,
> -				struct vm_area_struct *vma, pgoff_t index,
> -				loff_t write_end, bool shmem_huge_force)
> +		struct vm_area_struct *vma, vm_flags_t vm_flags,
> +		pgoff_t index, loff_t write_end, bool shmem_huge_force)
>  {
>  	unsigned long mask = READ_ONCE(huge_shmem_orders_always);
>  	unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
> -	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
>  	unsigned int global_orders;
>
>  	if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
> @@ -2430,6 +2429,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>  		gfp_t gfp, struct vm_fault *vmf, vm_fault_t *fault_type)
>  {
>  	struct vm_area_struct *vma = vmf ? vmf->vma : NULL;
> +	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
>  	struct mm_struct *fault_mm;
>  	struct folio *folio;
>  	int error;
> @@ -2507,7 +2507,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>  	}
>
>  	/* Find hugepage orders that are allowed for anonymous shmem and tmpfs. */
> -	orders = shmem_allowable_huge_orders(inode, vma, index, write_end, false);
> +	orders = shmem_allowable_huge_orders(inode, vma, vm_flags, index,
> +					     write_end, false);
>  	if (orders > 0) {
>  		gfp_t huge_gfp;
>
> --
> 2.47.3
>

--
Cheers, Lorenzo