Re: [PATCH v4 1/2] mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs

[James Houghton]

On Mon, Oct 12, 2026 at 12:00 PM Kiryl Shutsemau <kas@xxxxxxxxxx> wrote:
>
> On Sat, Oct 03, 2026 at 12:18:58AM +0000, James Houghton wrote:
> > Userfaultfd minor faults provides userspace with the ability to manually
> > install PTEs with UFFDIO_CONTINUE. Right now, MADV_COLLAPSE can map
> > holes in the VMA when a naturally-aligned THP is present. This is not
> > true for khugepaged collapse: the PTEs will be retracted, but a PMD will
> > not be installed.
> >
> > When MADV_COLLAPSE installs a PMD that mapped holes in the VMA,
> > userspace is likely to expect UFFDIO_CONTINUE to succeed on the
> > should-be holes. UFFDIO_CONTINUE will fail and return EEXIST.
> >
> > This is not inherently a problem, as MADV_COLLAPSE is an explicit
> > userspace action. But, especially because MADV_COLLAPSE can be invoked
> > by an external process via process_madvise(), a rogue caller could break
> > a userfaultfd-minor resolver thread.
> >
> > Userspace cannot generally use MADV_COLLAPSE to resolve userfault minor
> > faults, as MADV_COLLAPSE will only resolve such faults if a
> > naturally-aligned THP is present, so this is not a functional
> > regression for userspace.
> >
> > The naturally-aligned THP case is the only case where this quirk exists.
> > Collapsing otherwise requires all PTEs to be present for
> > userfaultfd-registered VMAs (i.e., max none PTEs is 0), which is
> > correct. This check is essentially bypassed for naturally-aligned THPs.
> >
> > Suggested-by: Lance Yang <lance.yang@xxxxxxxxx>
> > Tested-by: Lance Yang <lance.yang@xxxxxxxxx>
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> > Changes since v3:
> >   - Prevent page table retraction in patch 1. Please see the comment
> >     next to it. So this patch now has two hunks instead of one.
> >   - Reworded the comment and combined the userfaultfd checks in patch 1.
> >     (Thanks David)
> >   - Simplified the selftest a bit given the behavior change in patch 1.
> >   - Undid a change in v3's selftest that incorrectly handled the case
> >     where MADV_COLLAPSE was not supported entirely.
> >   - Rebased on top of mm-unstable, which includes Kiryl's changes.
> > v3: https://lore.kernel.org/linux-mm/20260910023411.514987-1-jthoughton@xxxxxxxxxx/
> > v2: https://lore.kernel.org/linux-mm/20260828222640.1638457-1-jthoughton@xxxxxxxxxx/
> > v1: https://lore.kernel.org/linux-mm/20260828005004.2870750-1-jthoughton@xxxxxxxxxx/
> > ---
> >  mm/khugepaged.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 913086eaf17b..438c4af1c058 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1868,10 +1868,11 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
> >               return SCAN_VMA_CHECK;
> >
> >       /*
> > -      * Keep pmd pgtable while the uffd bit is in use; see comment in
> > -      * retract_page_tables().
> > +      * Don't collapse if there might be PTE markers for userfaultfd-based
> > +      * access protection or if collapsing might bypass userfaultfd minor
> > +      * faults.
> >        */
> > -     if (userfaultfd_protected(vma))
> > +     if (userfaultfd_protected(vma) || userfaultfd_minor(vma))
> >               return SCAN_PTE_UFFD;
> >
> >       folio = filemap_lock_folio(vma->vm_file->f_mapping,
> > @@ -2093,8 +2094,13 @@ static bool file_backed_vma_is_retractable(struct vm_area_struct *vma)
> >        * and cannot be recycled to a shared PMD. Other vmas can still
> >        * have the same file mapped hugely, but skip this one: it will
> >        * always be mapped in small page size for these registrations.
> > +      *
> > +      * Userfaultfd-minor-registered VMAs should also not be retracted.
> > +      * PMDs will not be installed, as doing so can suppress minor faults.
> > +      * If retraction were allowed, khugepaged might continually cause
> > +      * unnecessary userfaultfd minor faults for already-CONTINUE'd pages.
> >        */
> > -     if (userfaultfd_protected(vma))
> > +     if (userfaultfd_protected(vma) || userfaultfd_minor(vma))
> >               return false;
> >
> >       /*
>
>
> Did you run 2/2 on current mm-unstable?
>
> Here, on e8d0f6a1b2a4 plus this series, minor-collapse fails every run:
>
>   # failed reason: [unexpected memory contents after collapse]
>   not ok 1 minor-collapse on shmem

Oh, I must have missed this when testing the unit test
"simplification" for this v4. :( Thanks for testing it.

It looks like uffd-wp can tolerate the try_to_unmap() because
cond_install_uffd_wp_ptes() will get called in the process.

> This patch refuses the collapse too late.
>
> collapse_scan_file() has no VMA, so the uffd check in
> collapse_max_ptes_none() never applies, and collapse_file() goes ahead:
> it try_to_unmap()s the PTEs userspace already CONTINUE'd and replaces
> the page cache.
>
> Only then does try_collapse_pte_mapped_thp() return SCAN_PTE_UFFD.
>
> MADV_COLLAPSE fails with -EINVAL, but the page the test read beforehand
> is already unmapped, takes a second minor fault, and gets flipped twice.

Thanks for explaining that, I didn't notice the try_to_unmap(). The v3
version of the selftest didn't have this problem because it only
attempted to access each page once.

The purpose of this patch is just to avoid installing PMDs; unmapping
PTEs is acceptable. I think the logic to truly prevent unmapping in
this case probably isn't worth it. Let me know if you disagree.

So I'll just go ahead with fixing the self-test, applying a diff like
this (sorry for the formatting), undoing the "simplification" from
v3->v4:

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c
b/tools/testing/selftests/mm/uffd-unit-tests.c
index 74ce43769e58..48435faa8bc8 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -553,6 +553,7 @@ static void
uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
  char c = '\0';
  struct uffd_args args = { .gopts = gopts };
  bool bad_contents;
+ int checked = 0;

  /*
  * NOTE: MADV_COLLAPSE is not yet compatible with WP, so testing
@@ -596,6 +597,7 @@ static void
uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
  * The page we already touched should not be unmapped, so a
  * second userfault on it should not be generated.
  */
+     checked = 1;
  }

  /*
@@ -604,7 +606,8 @@ static void
uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
  * fault. uffd_poll_thread will resolve the fault by bit-flipping the
  * page's contents, and then issuing a CONTINUE ioctl.
  */
- bad_contents = !!check_memory_contents(gopts, gopts->area_dst_alias);
+ bad_contents = !!__check_memory_contents(checked,
+         opts->nr_pages - checked, gopts, gopts->area_dst_alias);

> Also, your 2/2 missing Signed-off-by.

Thanks, I'll fix that.

I'm actually away right now, so I'll be slow responding to feedback
and can't send a new version until next week.

Thanks for your patience, Kiryl.