Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 22, 2021 at 12:29:35PM -0700, Yang Shi wrote:
> khugepqged does remove the pgtables. Please check out
> retract_page_tables(). The pmd will be cleared and the ptes will be
> freed otherwise the collapsed THP won't get PMD mapped by later
> access.

Indeed.

I should probably still properly disable khugepaged for at least VM_SHARED &&
VM_UFFD_WP, then I'd keep the anonymous && minor mode behavior untouched.

The other problem is even if current mm/vma doesn't have UFFD_WP registered,
some other mm/vma could have UFFD_WP enabled there that mapped the same file.
Checking that up within retract_page_tables() on all VMAs seems to be a bit too
late.

Checking it early may not trivially work too - I can walk the vma interval tree
at the entry of khugepaged_scan_file(), making sure no vma has UFFD_WP set.
However I don't see how it'll stop some of the vma from having UFFD_WP
registered later after that point but before retract_page_tables().

I'll need to think about it, but thanks for the input, Yang.  That's a very
important point.

-- 
Peter Xu