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

[Lance Yang <lance.yang@linux.dev>]

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'd leave that to the maintainers :D
>
>Anyway, assuming the answer is yes, I wonder whether the new check is
>sufficient. See below.
>
>>
>>If userspace implements post-copy live migration using userfaultfd minor
>>faults, this situation is currently possible:
>>1. The VMA for guest memory is userfaultfd-minor-registered and nothing
>>   is mapped in the page tables.
>>2. A stale copy of a page is present in a naturally-aligned THP (from
>>   pre-copy live migration).
>>3. khugepaged collapses the mapping of the THP, installs a PMD.

Ouch ... I missed this earlier. The problem is real, but this commit
message describes the wrong trigger.

Background khugepaged calls try_collapse_pte_mapped_thp() with
install_pmd=false, so it cannot install the PMD or trigger this
sequence. MADV_COLLAPSE passes install_pmd=true and installs the PMD.

So the problem described here can only be triggered by MADV_COLLAPSE,
whether it comes through madvise() or process_madvise(), no?

Cheers, Lance


>>4. The VM now has access to the stale contents => VM is broken.
>>5. After installing the correct contents, userspace attempts to map the
>>   page with UFFDIO_CONTINUE; it gets EEXIST, indicating that something
>>   unexpectedly mapped the page.
>>
>>The naturally-aligned THP case is the only case where this is a problem.
>>khugepaged 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.
>>
>>To deal with this issue, completely disallow collapsing in
>>userfaultfd-minor-registered VMAs. This is slightly pessimistic; it
>>would be nice to allow MADV_COLLAPSE to work if all PTEs are in fact
>>present, but that seems more complex than it is worth.
>>
>>Fixes: 58ac9a8993a1 ("mm/khugepaged: attempt to map file/shmem-backed pte-mapped THPs by pmds")
>>Cc: <stable@vger.kernel.org> # 6.1
>>Signed-off-by: James Houghton <jthoughton@google.com>
>>---
>>This was caught with manual review while diagnosing a related issue
>>that came up with in Google's live migration testing.
>>
>>I've uploaded a mostly-AI-generated reproducer here[1]. As long as
>>/sys/kernel/mm/transparent_hugepage/shmem_enabled is not set to 'deny',
>>the repro should work.
>>
>>[1] https://gist.github.com/48ca/d399bf534158e80241fb4937ef1ff664
>>---
>> mm/khugepaged.c | 9 +++++++++
>> 1 file changed, 9 insertions(+)
>>
>>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>index b237f6e7662a..66f956d3dd67 100644
>>--- a/mm/khugepaged.c
>>+++ b/mm/khugepaged.c
>>@@ -2804,6 +2804,15 @@ static enum scan_result collapse_single_pmd(unsigned long addr,
>> 		goto end;
>> 	}
>> 
>>+	/*
>>+	 * Userfaultfd-minor-registered VMAs should not be collapsed, as
>>+	 * userspace is expecting to explicitly install PTEs.
>>+	 */
>>+	if (userfaultfd_minor(vma)) {
>>+		result = SCAN_PTE_UFFD;
>>+		goto end;
>>+	}
>
>Assume UFFDIO_REGISTER_MODE_MINOR completes after collapse_single_pmd()
>drops the mmap read lock and before it reacquires it.
>
>Doesn't this still leave a registration race, no?
>
>
>int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>		     unsigned long end, bool *lock_dropped)
>{
>...
>	cc->is_khugepaged = false;
>...
>		result = collapse_single_pmd(addr, vma, &mmap_unlocked, cc);
>...
>}
>
>static enum scan_result collapse_single_pmd(unsigned long addr,
>		struct vm_area_struct *vma, bool *lock_dropped,
>		struct collapse_control *cc)
>{
>...
>	if (userfaultfd_minor(vma)) {
>		result = SCAN_PTE_UFFD;
>		goto end;
>	}
>...
>	mmap_read_unlock(mm);
>	*lock_dropped = true;
>...
>	if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
>		mmap_read_lock(mm);
>		if (collapse_test_exit_or_disable(mm))
>			result = SCAN_ANY_PROCESS;
>		else
>			result = try_collapse_pte_mapped_thp(mm, addr,
>							     !cc->is_khugepaged);
>...
>		mmap_read_unlock(mm);
>	}
>...
>}
>
>static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>		bool install_pmd)
>{
>...
>	struct vm_area_struct *vma = vma_lookup(mm, haddr);
>...
>	if (!vma || !vma->vm_file ||
>	    !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
>		return SCAN_VMA_CHECK;
>...
>	if (userfaultfd_protected(vma))
>		return SCAN_PTE_UFFD;
>...
>	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
>	switch (result) {
>	case SCAN_SUCCEED:
>		break;
>	case SCAN_NO_PTE_TABLE:
>...
>		goto maybe_install_pmd;
>	default:
>		goto drop_folio;
>	}
>...
>maybe_install_pmd:
>	/* step 5: install pmd entry */
>	result = install_pmd
>			? set_huge_pmd(vma, haddr, pmd, folio, &folio->page)
>			: SCAN_SUCCEED;
>...
>}
>
>
>static inline bool userfaultfd_minor(struct vm_area_struct *vma)
>{
>	return vma_test_any_mask(vma, VMA_UFFD_MINOR);
>}
>
>static inline bool userfaultfd_protected(struct vm_area_struct *vma)
>{
>	return userfaultfd_wp(vma) || userfaultfd_rwp(vma);
>}
>
>Emm ... userfaultfd_protected() only covers WP and RWP. MADV_COLLAPSE
>passes install_pmd=true, so the SCAN_NO_PTE_TABLE case can still reach
>set_huge_pmd() after UFFDIO_REGISTER_MODE_MINOR has completed ...
>
>Maybe:
>
>---8<---
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 33c41bc32af8..0eada7265d59 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -1893,6 +1893,8 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
> 	 */
> 	if (userfaultfd_protected(vma))
> 		return SCAN_PTE_UFFD;
>+	if (userfaultfd_minor(vma))
>+		return SCAN_PTE_UFFD;
>
> 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
> 			       linear_page_index(vma, haddr));
>--
>
>With that, LGTM.
>
>Tested-by: Lance Yang <lance.yang@linux.dev>
>
>Cheers, Lance
>