[PATCH v10 02/13] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

Nico Pache
2025-08-19T13:43:43Z

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.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 94 ++++++++++++++++++++++++++-----------------------
1 file changed, 49 insertions(+), 45 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0e7bbadf03ee..b7b98aebb670 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2382,6 +2382,50 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
return result;
}

+/*
+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
+ * the results.
+ */
+static int collapse_single_pmd(unsigned long addr,
+ struct vm_area_struct *vma, bool *mmap_locked,
+ struct collapse_control *cc)
+{
+ int result = SCAN_FAIL;
+ struct mm_struct *mm = vma->vm_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_scan_file(mm, addr, file, pgoff, cc);
+ fput(file);
+ if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
+ mmap_read_lock(mm);
+ *mmap_locked = true;
+ if (collapse_test_exit_or_disable(mm)) {
+ mmap_read_unlock(mm);
+ *mmap_locked = false;
+ result = SCAN_ANY_PROCESS;
+ goto end;
+ }
+ result = collapse_pte_mapped_thp(mm, addr,
+ !cc->is_khugepaged);
+ if (result == SCAN_PMD_MAPPED)
+ result = SCAN_SUCCEED;
+ mmap_read_unlock(mm);
+ *mmap_locked = false;
+ }
+ } else {
+ result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
+ }
+ if (cc->is_khugepaged && result == SCAN_SUCCEED)
+ ++khugepaged_pages_collapsed;
+end:
+ return result;
+}
+
static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
struct collapse_control *cc)
__releases(&khugepaged_mm_lock)
@@ -2455,34 +2499,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *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 = 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;
@@ -2799,34 +2818,19 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
mmap_assert_locked(mm);
memset(cc->node_load, 0, sizeof(cc->node_load));
nodes_clear(cc->alloc_nmask);
- 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_scan_file(mm, addr, file, pgoff, cc);
- fput(file);
- } else {
- result = collapse_scan_pmd(mm, vma, addr,
- &mmap_locked, cc);
- }
+ result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
+
if (!mmap_locked)
*lock_dropped = true;

-handle_result:
switch (result) {
case SCAN_SUCCEED:
case SCAN_PMD_MAPPED:
++thps;
break;
- case SCAN_PTE_MAPPED_HUGEPAGE:
- BUG_ON(mmap_locked);
- mmap_read_lock(mm);
- result = collapse_pte_mapped_thp(mm, addr, true);
- mmap_read_unlock(mm);
- goto handle_result;
/* Whitelisted set of results where continuing OK */
+ case SCAN_PTE_MAPPED_HUGEPAGE:
case SCAN_PMD_NULL:
case SCAN_PTE_NON_PRESENT:
case SCAN_PTE_UFFD_WP:
--
2.50.1