Re: [PATCH] mm: prevent droppable mappings from being locked

[anthony.yznaga@oracle.com]

On 3/9/26 7:31 AM, Lorenzo Stoakes (Oracle) wrote:
> On Mon, Mar 09, 2026 at 03:15:24PM +0100, David Hildenbrand (Arm) wrote:
>> On 3/6/26 21:45, Anthony Yznaga wrote:
>>> Mappings created with MAP_DROPPABLE cannot be locked via mlock() due
>>> to the check in mlock_fixup(). However, they will be locked indirectly
>>> if they are created after mlockall(MCL_FUTURE).
>>>
>>> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
>>> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
>>> ---
>>>   include/linux/mm.h | 3 +++
>>>   mm/mlock.c         | 4 ++--
>>>   mm/vma.c           | 2 +-
>>>   3 files changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 5be3d8a8f806..bb830574d112 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -574,6 +574,9 @@ enum {
>>>   /* This mask represents all the VMA flag bits used by mlock */
>>>   #define VM_LOCKED_MASK	(VM_LOCKED | VM_LOCKONFAULT)
>>>
>>> +/* This mask prevents VMAs from being mlock'd */
>>> +#define VM_NO_MLOCK_MASK	(VM_SPECIAL | VM_DROPPABLE)
>> Instead of adding that, could we cleanup further by doing something like the following?
>>
>> The usage of "vma->vm_mm" must be double checked, and we'll have to take care of making
>> the tools/testing/vma test happy.
> Yeah Anthony - please do a simple:
>
> $ cd tools/testing/vma
> $ make && ./vma
>
> To make sure that your changes don't introduce anything that breaks that.
>
> If you need to add duplicate header defines put them in
> tools/testing/vma/include/dup.h, if you need to stub stuff out put in stubs.h
> and if you need to customise something for testing purposes, put it in custom.h.

Thanks, Lorenzo. Much appreciated!

Anthony


>
>> Not even compile tested, so will require some more work.
>>
>>
>> diff --git a/include/linux/hugetlb_inline.h b/include/linux/hugetlb_inline.h
>> index 593f5d4e108b..755281fab23d 100644
>> --- a/include/linux/hugetlb_inline.h
>> +++ b/include/linux/hugetlb_inline.h
>> @@ -30,7 +30,7 @@ static inline bool is_vma_hugetlb_flags(const vma_flags_t *flags)
>>
>>   #endif
>>
>> -static inline bool is_vm_hugetlb_page(struct vm_area_struct *vma)
>> +static inline bool is_vm_hugetlb_page(const struct vm_area_struct *vma)
>>   {
>>   	return is_vm_hugetlb_flags(vma->vm_flags);
>>   }
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 6e1162e13289..b70ebbdafe00 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1242,6 +1242,15 @@ static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
>>   	}
>>   	return fpin;
>>   }
>> +
>> +static inline bool vma_supports_mlock(const struct vm_area_struct *vma)
>> +{
>> +	if (vma->vm_flags & (VM_SPECIAL | VM_DROPPABLE))
>> +		return false;
>> +	if (vma_is_dax(vma) || is_vm_hugetlb_page(vma))
>> +		return false;
>> +	return vma != get_gate_vma(vma->vm_mm);
>> +}
> Yeah this is nice.
>
>>   #else /* !CONFIG_MMU */
>>   static inline void unmap_mapping_folio(struct folio *folio) { }
>>   static inline void mlock_new_folio(struct folio *folio) { }
>> diff --git a/mm/mlock.c b/mm/mlock.c
>> index 1a92d16f3684..e16b2ea234f7 100644
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -472,9 +472,7 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>>   	int ret = 0;
>>   	vm_flags_t oldflags = vma->vm_flags;
>>
>> -	if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
>> -	    is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
>> -	    vma_is_dax(vma) || vma_is_secretmem(vma) || (oldflags & VM_DROPPABLE))
>> +	if (newflags == oldflags || !vma_supports_mlock(vma))
>>   		/* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
>>   		goto out;
>>
>> diff --git a/mm/vma.c b/mm/vma.c
>> index e95fd5a5fe5c..b7055c264b5d 100644
>> --- a/mm/vma.c
>> +++ b/mm/vma.c
>> @@ -2589,9 +2589,7 @@ static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma)
>>
>>   	vm_stat_account(mm, vma->vm_flags, map->pglen);
>>   	if (vm_flags & VM_LOCKED) {
>> -		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
>> -					is_vm_hugetlb_page(vma) ||
>> -					vma == get_gate_vma(mm))
>> +		if (!vma_supports_mlock(vma))
>>   			vm_flags_clear(vma, VM_LOCKED_MASK);
>>   		else
>>   			mm->locked_vm += map->pglen;
> Very much preferable!
>
>> --
>> 2.43.0
>>
>> --
>> Cheers,
>>
>> David
> Cheers, Lorenzo