Andrew - when this goes into mm-new if there isn't a respin between, please
drop all tags except any obviously sent re: the fix-patch.

Thanks!

On Fri, Jan 23, 2026 at 01:07:16PM +0800, Lance Yang wrote:

On 2026/1/23 03:28, Nico Pache wrote:

quoted

The khugepaged daemon and madvise_collapse have two different
implementations that do almost the same thing.

Create collapse_single_pmd to increase code reuse and create an entry
point to these two users.

Refactor madvise_collapse and collapse_scan_mm_slot to use the new
collapse_single_pmd function. This introduces a minor behavioral change
that is most likely an undiscovered bug. The current implementation of
khugepaged tests collapse_test_exit_or_disable before calling
collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
case. By unifying these two callers madvise_collapse now also performs
this check. We also modify the return value to be SCAN_ANY_PROCESS which
properly indicates that this process is no longer valid to operate on.

We also guard the khugepaged_pages_collapsed variable to ensure its only
incremented for khugepaged.

Reviewed-by: Wei Yang <redacted>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lorenzo Stoakes <redacted>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <redacted>
Signed-off-by: Nico Pache <npache@redhat.com>
---

I think this patch introduces some functional changes compared to previous
version[1] ...

Maybe we should drop the r-b tags and let folks take another look?

Yes thanks Lance, absolutely this should happen.

Especially on a small-iteration respin (I really wanted to get to v13 but the
rebase issue killed that).

I know it wasn't intentional, not suggesting that of course :) just obviously as
a process thing - it's _very_ important to make clear what you've changed and
what you haven't. For truly minor changes no need to drop the tags, but often my
workflow is:

- Check which patches I haven't reviewed yet.
- Go review those.

So I might well have missed that.

I often try to do a git range-diff, but in this case I probably wouldn't have on
basis of the v13 having merge conflicts.

But obviously given the above I went and fixed them up and applied v13 locally
so I could check everything :)

There might be an issue with the vma access in madvise_collapse(). See
below:

[1]
https://lore.kernel.org/linux-mm/20251201174627.23295-3-npache@redhat.com/ (local)

quoted

  mm/khugepaged.c | 106 +++++++++++++++++++++++++++---------------------
  1 file changed, 60 insertions(+), 46 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fefcbdca4510..59e5a5588d85 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -2394,6 +2394,54 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, unsigned long a
  	return result;
  }
+/*
+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
+ * the results.
+ */
+static enum scan_result collapse_single_pmd(unsigned long addr,
+		struct vm_area_struct *vma, bool *mmap_locked,
+		struct collapse_control *cc)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	enum scan_result result;
+	struct file *file;
+	pgoff_t pgoff;
+
+	if (vma_is_anonymous(vma)) {
+		result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
+		goto end;
+	}
+
+	file = get_file(vma->vm_file);
+	pgoff = linear_page_index(vma, addr);
+
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+	result = collapse_scan_file(mm, addr, file, pgoff, cc);
+	fput(file);
+
+	if (result != SCAN_PTE_MAPPED_HUGEPAGE)
+		goto end;
+
+	mmap_read_lock(mm);
+	*mmap_locked = true;
+	if (collapse_test_exit_or_disable(mm)) {
+		mmap_read_unlock(mm);
+		*mmap_locked = false;
+		return SCAN_ANY_PROCESS;
+	}
+	result = try_collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
+	if (result == SCAN_PMD_MAPPED)
+		result = SCAN_SUCCEED;
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+
+end:
+	if (cc->is_khugepaged && result == SCAN_SUCCEED)
+		++khugepaged_pages_collapsed;
+	return result;
+}
+
  static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *result,
  					    struct collapse_control *cc)
  	__releases(&khugepaged_mm_lock)

@@ -2466,34 +2514,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *
  			VM_BUG_ON(khugepaged_scan.address < hstart ||
  				  khugepaged_scan.address + HPAGE_PMD_SIZE >
  				  hend);
-			if (!vma_is_anonymous(vma)) {
-				struct file *file = get_file(vma->vm_file);
-				pgoff_t pgoff = linear_page_index(vma,
-						khugepaged_scan.address);
-
-				mmap_read_unlock(mm);
-				mmap_locked = false;
-				*result = collapse_scan_file(mm,
-					khugepaged_scan.address, file, pgoff, cc);
-				fput(file);
-				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
-					mmap_read_lock(mm);
-					if (collapse_test_exit_or_disable(mm))
-						goto breakouterloop;
-					*result = try_collapse_pte_mapped_thp(mm,
-						khugepaged_scan.address, false);
-					if (*result == SCAN_PMD_MAPPED)
-						*result = SCAN_SUCCEED;
-					mmap_read_unlock(mm);
-				}
-			} else {
-				*result = collapse_scan_pmd(mm, vma,
-					khugepaged_scan.address, &mmap_locked, cc);
-			}
-
-			if (*result == SCAN_SUCCEED)
-				++khugepaged_pages_collapsed;
+			*result = collapse_single_pmd(khugepaged_scan.address,
+						      vma, &mmap_locked, cc);
  			/* move to next address */
  			khugepaged_scan.address += HPAGE_PMD_SIZE;
  			progress += HPAGE_PMD_NR;

@@ -2799,6 +2822,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
  			cond_resched();
  			mmap_read_lock(mm);
  			mmap_locked = true;
+			*lock_dropped = true;
  			result = hugepage_vma_revalidate(mm, addr, false, &vma,
  							 cc);
  			if (result  != SCAN_SUCCEED) {

@@ -2809,17 +2833,17 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
  			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
  		}
  		mmap_assert_locked(mm);
-		if (!vma_is_anonymous(vma)) {
-			struct file *file = get_file(vma->vm_file);
-			pgoff_t pgoff = linear_page_index(vma, addr);
-			mmap_read_unlock(mm);
-			mmap_locked = false;
+		result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
+
+		if (!mmap_locked)
  			*lock_dropped = true;
-			result = collapse_scan_file(mm, addr, file, pgoff, cc);
-			if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
-			    mapping_can_writeback(file->f_mapping)) {
+		if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
+			struct file *file = get_file(vma->vm_file);
+			pgoff_t pgoff = linear_page_index(vma, addr);

After collapse_single_pmd() returns, mmap_lock might have been released.
Between
that unlock and here, another thread could unmap/remap the VMA, making the
vma
pointer stale when we access vma->vm_file?

Yeah, yikes.

The locking logic around this code is horrifying... but that's one for future
series I guess.

Would it be safer to get the file reference before calling
collapse_single_pmd()?
Or we need to revalidate the VMA after getting the lock back?

Also obviously the pgoff.

I know Nico suggested a patch in a response, will check.

Cheers, Lorenzo