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

On 8/17/26 3:40 PM, Lorenzo Stoakes (ARM) wrote:
> 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.

Yes.

>> 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.

Sure.


>> 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.

This is not the point (maybe I didn't describe it clearly). The point is 
that the allowable orders check for tmpfs in shmem_huge_global_enabled() 
(called by shmem_allowable_huge_orders()) uses a stale vma flag.

static unsigned int shmem_huge_global_enabled(struct inode *inode, 
pgoff_t index,
			loff_t write_end, bool shmem_huge_force,
			struct vm_area_struct *vma,
			vm_flags_t vm_flags)
{
	......
	switch (SHMEM_SB(inode->i_sb)->huge) {

	......
	case SHMEM_HUGE_ADVISE:
		if (vm_flags & VM_HUGEPAGE)
			return THP_ORDERS_ALL_FILE_DEFAULT;
		fallthrough;
	default:
		return 0;
	}
}

So we should pass the new vma flags for shmem_allowable_huge_orders() to 
check the allowable orders for tmpfs. Changing madvise_update_vma() 
doesn't help with the allowable orders check for tmpfs.

> 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 :)

Great. Look forward to seeing these cleanups.

>> Reported-by: Lance Yang <[email protected]>
>> Closes: https://lore.kernel.org/all/[email protected]/
>> Fixes: 6beeab870e70 ("mm: shmem: move shmem_huge_global_enabled() into shmem_allowable_huge_orders()")
>> Cc: [email protected]
>> Signed-off-by: Baolin Wang <[email protected]>
>> ---
>>   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