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

[anthony . yznaga]

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@xxxxxxxxxx>
> > > ---
> > >   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