Re: [PATCH] mm/khugepaged: Don't collapse uffd-minor-registered VMAs

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Aug 28, 2026 at 05:47:03PM +0800, Lance Yang wrote:
> 
> On Fri, Aug 28, 2026 at 12:50:04AM +0000, James Houghton wrote:
> >Userfaultfd minor faults provides userspace with the ability to manually
> >install PTEs with UFFDIO_CONTINUE. Right now, khugepaged collapse can
> >map holes in the VMA when a naturally-aligned THP is present without
> >explicit action from userspace.
> >
> >This is a problem, as it bypasses userfaultfd minor faults that
> >userspace is expecting to handle.
> 
> One basic question first. Should MADV_COLLAPSE refuse to collapse a
> UFFD-minor-registered VMA, regardless of whether all PTEs are present?

I think the line is not between MADV_COLLAPSE vs khugepaged, it is if
vma->vm_mm is current->mm or not.

In other words, we might want to allow collapse if workload itself asks
for it, but not for khugepaged or *external* process_madvise(MADV_COLLAPSE).
is_khugepaged is not the right proxy.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov