Re: [PATCH 02/13] mm/vma: update do_mmap() to use vma_flags_t

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Jul 06, 2026 at 10:10:32PM -0400, Zi Yan wrote:
> On Mon Jun 29, 2026 at 3:25 PM EDT, Lorenzo Stoakes wrote:
> > The core do_mmap() function accepts a vm_flags_t parameter which it then
> > manipulates before passing to mmap_region() to do the heavy lifting of the
> > memory mapping.
> >
> > Update do_mmap() to instead accept a vma_flags_t parameter, and adjust all
> > the logic within do_mmap() to manipulate this instead.
> >
> > This is as part of the ongoing effort to convert VMA flags from a system
> > word size to a bitmap type which allows us to unrestrict the number of VMA
> > flags, as well as gain control over how VMA flag manipulation occurs.
> >
> > We do not cascade these changes to all functions which accept vm_flags_t,
> > but rather use vma_flags_to_legacy() where necessary, specifically
> > deferring converting calc_vm_prot_bits(), calc_vm_flag_bits() and
> > __get_unmapped_area() to vma_flags_t.
> >
> > Also utilise the new vma_flags_can_grow() predicate which correctly handles
> > the case of architectures without upward growing stacks.
> >
> > As part of this change, introduce VMA_SHADOW_STACK so we can correctly
> > handle the case of the shadow stack not being defined.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
> > ---
> >  arch/mips/kernel/vdso.c |  4 +--
> >  fs/aio.c                |  2 +-
> >  include/linux/memfd.h   |  6 ++--
> >  include/linux/mm.h      |  6 ++--
> >  ipc/shm.c               |  3 +-
> >  mm/memfd.c              | 15 ++++-----
> >  mm/mmap.c               | 67 ++++++++++++++++++++++++-----------------
> >  mm/nommu.c              |  3 +-
> >  mm/util.c               | 10 +++---
> >  mm/vma.c                |  7 ++---
> >  mm/vma.h                |  2 +-
> >  11 files changed, 69 insertions(+), 56 deletions(-)
> >
>
> <snip>
>
> >
> > -static int check_write_seal(vm_flags_t *vm_flags_ptr)
> > +static int check_write_seal(vma_flags_t *vma_flags_ptr)
> >  {
> > -	vm_flags_t vm_flags = *vm_flags_ptr;
> > -	vm_flags_t mask = vm_flags & (VM_SHARED | VM_WRITE);
> > -
> >  	/* If a private mapping then writability is irrelevant. */
> > -	if (!(mask & VM_SHARED))
> > +	if (!vma_flags_test(vma_flags_ptr, VMA_SHARED_BIT))
> >  		return 0;
> >
> >  	/*
> >  	 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
> >  	 * write seals are active.
> >  	 */
> > -	if (mask & VM_WRITE)
> > +	if (vma_flags_test(vma_flags_ptr, VMA_WRITE_BIT))
> >  		return -EPERM;
> >
> >  	/*
> >  	 * This is a read-only mapping, disallow mprotect() from making a
> >  	 * write-sealed mapping writable in future.
> >  	 */
> > -	*vm_flags_ptr &= ~VM_MAYWRITE;
> > +	vma_flags_clear(vma_flags_ptr, VMA_MAYWRITE_BIT);
> >
> >  	return 0;
> >  }
>
> This function alone changed its original behavior, since vm_flags is a
> snapshot of *vm_flags_ptr, but after the change this snapshot is gone.
> But its only caller memfd_check_seals_mmap() gets vm_flags_ptr from the
> input parameter of do_mmap(), so the overall behavior does not change.

Right yeah, the snapshot was always just a convenience thing :)

>
> <snip>
>
> > +		case MAP_DROPPABLE: {
> > +			vma_flags_t droppable = VMA_DROPPABLE;
> > +
> > +			if (vma_flags_empty(&droppable))
> >  				return -EOPNOTSUPP;
> > +			vma_flags_set_mask(&vma_flags, droppable);
> > +
> >  			/*
> >  			 * A locked or stack area makes no sense to be droppable.
> >  			 *
> > @@ -515,23 +527,24 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> >  			 */
> >  			if (flags & (MAP_LOCKED | MAP_HUGETLB))
> >  			        return -EINVAL;
> > -			if (vm_flags & (VM_GROWSDOWN | VM_GROWSUP))
> > +			if (vma_flags_can_grow(&vma_flags))
> >  			        return -EINVAL;
> >
> > -			vm_flags |= VM_DROPPABLE;
> > -
>
> Lance pointed out the reordering of setting VMA_DROPPABLE and checking
> of can_grow, but these flags are not overlapped and there is no parallel
> writer to vma_flags. So it is still no functional change, just not
> mechanical changes. :)

Right yes exactly :)

>
> Otherwise, LGTM.
>
> Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks!

>
> --
> Best Regards,
> Yan, Zi
>

Cheers, Lorenzo