Messages in this thread |  | | Date | Fri, 14 Feb 2020 03:36:40 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap(). |
| |
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
|  |