Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Feb 13, 2020 at 10:20:44AM -0800, Brian Geffon wrote:
> Hi Kirill,
> 
> > But if you do the operation for the VM_LOCKED vma, you'll have two locked
> > VMA's now, right? Where do you account the old locked vma you left behind?
> 
> You bring up a good point. In a previous iteration of my patch I had
> it clearing the locked flags on the old VMA as technically the locked
> pages had migrated. I talked myself out of that but the more I think
> about it we should probably do that. Something along the lines of:
> 
> +    if (vm_flags & VM_LOCKED) {
> +      /* Locked pages would have migrated to the new VMA */
> +      vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
> +      if (new_len > old_len)
> +              mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT;
> +   }
> 
> I feel that this is correct. The only other possible option would be
> to clear only the VM_LOCKED flag on the old vma leaving VM_LOCKONFAULT
> to handle the MCL_ONFAULT mlocked situation, thoughts? Regardless I'll
> have to mail a new patch because that part where I'm incrementing the
> mm->locked_vm lost the check on VM_LOCKED during patch versions.

Note, that we account mlock limit on per-VMA basis, not per page. Even for
VM_LOCKONFAULT.

> Thanks again for taking the time to review.

I believe the right approach is to strip VM_LOCKED[ONFAULT] from the vma
you left behind. Or the new vma. It is a policy decision.

JFYI, we do not inherit VM_LOCKED on fork(), so it's common practice to
strip VM_LOCKED on vma duplication.

Other option is to leave VM_LOCKED on both VMAs and fail the operation if
we are over the limit. But we need to have a good reason to take this
path. It makes the interface less flexible.

-- 
 Kirill A. Shutemov