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

[anthony.yznaga@oracle.com]

On 3/9/26 7:15 AM, 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.
>
> Not even compile tested, so will require some more work.

Thanks, David. This is a better approach that I'll implement. One thing 
to note is that the check for secretmem has to stay in mlock_fixup() 
because it's preventing the always-locked memory from being unlocked. I 
can add an extra comment for that.

Anthony

>
>
> 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);
> +}
>   #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;