Re: [PATCH mm-new v4 2/6] mm: khugepaged: refine scan progress number

[Vernon Yang <vernon2gm@gmail.com>]

On Wed, Jan 14, 2026 at 12:38:46PM +0100, David Hildenbrand (Red Hat) wrote:
> On 1/11/26 13:19, Vernon Yang wrote:
> > Currently, each scan always increases "progress" by HPAGE_PMD_NR,
> > even if only scanning a single pte.
> >
> > This patch does not change the original semantics of "progress", it
> > simply uses the exact number of PTEs counted to replace HPAGE_PMD_NR.
> >
> > Let me provide a detailed example:
> >
> > static int hpage_collapse_scan_pmd()
> > {
> > 	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> > 	     _pte++, addr += PAGE_SIZE) {
> > 		pte_t pteval = ptep_get(_pte);
> > 		...
> > 		if (pte_uffd_wp(pteval)) { <-- first scan hit
> > 			result = SCAN_PTE_UFFD_WP;
> > 			goto out_unmap;
> > 		}
> > 	}
> > }
> >
> > During the first scan, if pte_uffd_wp(pteval) is true, the loop exits
> > directly. In practice, only one PTE is scanned before termination.
> > Here, "progress += 1" reflects the actual number of PTEs scanned, but
> > previously "progress += HPAGE_PMD_NR" always.
> >
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >   mm/khugepaged.c | 28 ++++++++++++++++++++++------
> >   1 file changed, 22 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2e570f83778c..5c6015ac7b5e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1249,6 +1249,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> >   static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >   						struct vm_area_struct *vma,
> >   						unsigned long start_addr, bool *mmap_locked,
> > +						int *cur_progress,
> >   						struct collapse_control *cc)
> >   {
> >   	pmd_t *pmd;
> > @@ -1264,19 +1265,27 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >   	VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
> >   	result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
> > -	if (result != SCAN_SUCCEED)
> > +	if (result != SCAN_SUCCEED) {
> > +		if (cur_progress)
> > +			*cur_progress = HPAGE_PMD_NR;
> >   		goto out;
> > +	}
> >   	memset(cc->node_load, 0, sizeof(cc->node_load));
> >   	nodes_clear(cc->alloc_nmask);
> >   	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
> >   	if (!pte) {
> > +		if (cur_progress)
> > +			*cur_progress = HPAGE_PMD_NR;
> >   		result = SCAN_NO_PTE_TABLE;
> >   		goto out;
> >   	}
> >   	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> >   	     _pte++, addr += PAGE_SIZE) {
> > +		if (cur_progress)
> > +			*cur_progress += 1;
> > +
> >   		pte_t pteval = ptep_get(_pte);
> >   		if (pte_none_or_zero(pteval)) {
> >   			++none_or_zero;
> > @@ -2297,6 +2306,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >   static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> >   						 struct file *file, pgoff_t start,
> > +						 int *cur_progress,
> >   						 struct collapse_control *cc)
> >   {
> >   	struct folio *folio = NULL;
> > @@ -2337,6 +2347,9 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
> >   			continue;
> >   		}
> > +		if (cur_progress)
> > +			*cur_progress += folio_nr_pages(folio);
> > +
>
> Okay, I had another look and I think the file path is confusing. We're
> scanning xarray entries. But then, we only count some entries and not
> others.

Thank you for the review.

Move PATCH #3 comments here

	> Assume we found a single 4k folio in the xarray, but then collapse a 2M THP.
	> Is the progress really "1" ?

	This example is indeed a issue, I will use "xas->xa_index" to fix
	these issues.

	> What about shmem swap entries (xa_is_value)?

	Sorry, I missed it. I will add "1 << xas_get_order(&xas)" to
	"cur_progreess".

> Can we just keep that alone in this patch? That is, always indicate a
> progress of HPAGE_PMD_NR right at the start of the function?

Studying the implementation source code of xarray, I discovered that
these issues can be fixed by utilizing "xas->xa_index".

I send the code as follow (patch #2 and #3 are squashed). Let's see if
it works? If not, please let me know, Thanks!

--
Thanks,
Vernon


diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index be850174e802..f77d97d7b957 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1646,6 +1646,15 @@ static inline void xas_set(struct xa_state *xas, unsigned long index)
 	xas->xa_node = XAS_RESTART;
 }

+/**
+ * xas_get_index() - Get XArray operation state for a different index.
+ * @xas: XArray operation state.
+ */
+static inline unsigned long xas_get_index(struct xa_state *xas)
+{
+	return xas->xa_index;
+}
+
 /**
  * xas_advance() - Skip over sibling entries.
  * @xas: XArray operation state.
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 2e570f83778c..7d42035ece5b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -68,7 +68,10 @@ enum scan_result {
 static struct task_struct *khugepaged_thread __read_mostly;
 static DEFINE_MUTEX(khugepaged_mutex);

-/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
+/*
+ * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
+ * every 10 second.
+ */
 static unsigned int khugepaged_pages_to_scan __read_mostly;
 static unsigned int khugepaged_pages_collapsed;
 static unsigned int khugepaged_full_scans;
@@ -1249,6 +1252,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
 						struct vm_area_struct *vma,
 						unsigned long start_addr, bool *mmap_locked,
+						unsigned int *cur_progress,
 						struct collapse_control *cc)
 {
 	pmd_t *pmd;
@@ -1263,6 +1267,9 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,

 	VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);

+	if (cur_progress)
+		*cur_progress += 1;
+
 	result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
 	if (result != SCAN_SUCCEED)
 		goto out;
@@ -1403,6 +1410,8 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
 	} else {
 		result = SCAN_SUCCEED;
 	}
+	if (cur_progress)
+		*cur_progress += _pte - pte;
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
@@ -2297,6 +2306,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,

 static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 						 struct file *file, pgoff_t start,
+						 unsigned int *cur_progress,
 						 struct collapse_control *cc)
 {
 	struct folio *folio = NULL;
@@ -2386,6 +2396,18 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
 			cond_resched_rcu();
 		}
 	}
+	if (cur_progress) {
+		unsigned long idx = xas_get_index(&xas) - start;
+
+		if (folio == NULL)
+			*cur_progress += HPAGE_PMD_NR;
+		else if (xa_is_value(folio))
+			*cur_progress += idx + (1 << xas_get_order(&xas));
+		else if (folio_order(folio) == HPAGE_PMD_ORDER)
+			*cur_progress += idx + 1;
+		else
+			*cur_progress += idx + folio_nr_pages(folio);
+	}
 	rcu_read_unlock();

 	if (result == SCAN_SUCCEED) {
@@ -2466,6 +2488,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result

 		while (khugepaged_scan.address < hend) {
 			bool mmap_locked = true;
+			unsigned int cur_progress = 0;

 			cond_resched();
 			if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
@@ -2482,7 +2505,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
 				mmap_read_unlock(mm);
 				mmap_locked = false;
 				*result = hpage_collapse_scan_file(mm,
-					khugepaged_scan.address, file, pgoff, cc);
+					khugepaged_scan.address, file, pgoff,
+					&cur_progress, cc);
 				fput(file);
 				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
 					mmap_read_lock(mm);
@@ -2496,7 +2520,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
 				}
 			} else {
 				*result = hpage_collapse_scan_pmd(mm, vma,
-					khugepaged_scan.address, &mmap_locked, cc);
+					khugepaged_scan.address, &mmap_locked,
+					&cur_progress, cc);
 			}

 			if (*result == SCAN_SUCCEED)
@@ -2504,7 +2529,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result

 			/* move to next address */
 			khugepaged_scan.address += HPAGE_PMD_SIZE;
-			progress += HPAGE_PMD_NR;
+			progress += cur_progress;
 			if (!mmap_locked)
 				/*
 				 * We released mmap_lock so break loop.  Note
@@ -2826,11 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			mmap_read_unlock(mm);
 			mmap_locked = false;
 			result = hpage_collapse_scan_file(mm, addr, file, pgoff,
-							  cc);
+							  NULL, cc);
 			fput(file);
 		} else {
 			result = hpage_collapse_scan_pmd(mm, vma, addr,
-							 &mmap_locked, cc);
+							 &mmap_locked, NULL, cc);
 		}
 		if (!mmap_locked)
 			*lock_dropped = true;