[PATCH mm-unstable v3 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

Nico Pache
2026-03-11T21:16:39Z

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properly indicates that this process is no longer valid to operate on.

By moving the madvise_collapse writeback-retry logic into the helper
function we can also avoid having to revalidate the VMA.

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

Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 120 +++++++++++++++++++++++++-----------------------
1 file changed, 63 insertions(+), 57 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 33ae56e313ed..733c4a42c2ce 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2409,6 +2409,65 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
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;
+ bool triggered_wb = false;
+ 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;
+retry:
+ result = collapse_scan_file(mm, addr, file, pgoff, cc);
+
+ /*
+ * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
+ * then retry the collapse one time.
+ */
+ if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
+ !triggered_wb && mapping_can_writeback(file->f_mapping)) {
+ const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
+ const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
+
+ filemap_write_and_wait_range(file->f_mapping, lstart, lend);
+ triggered_wb = true;
+ goto retry;
+ }
+ fput(file);
+
+ 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);
+ if (result == SCAN_PMD_MAPPED)
+ result = SCAN_SUCCEED;
+ mmap_read_unlock(mm);
+ }
+end:
+ if (cc->is_khugepaged && result == SCAN_SUCCEED)
+ ++khugepaged_pages_collapsed;
+ return result;
+}
+
static void collapse_scan_mm_slot(unsigned int progress_max,
enum scan_result *result, struct collapse_control *cc)
__releases(&khugepaged_mm_lock)
@@ -2479,34 +2538,9 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
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;
if (!mmap_locked)
@@ -2806,9 +2840,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,

for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
enum scan_result result = SCAN_FAIL;
- bool triggered_wb = false;

-retry:
if (!mmap_locked) {
cond_resched();
mmap_read_lock(mm);
@@ -2823,46 +2855,20 @@ 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;
- *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)) {
- loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
- loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
+ result = collapse_single_pmd(addr, vma, &mmap_locked, cc);

- filemap_write_and_wait_range(file->f_mapping, lstart, lend);
- triggered_wb = true;
- fput(file);
- goto retry;
- }
- fput(file);
- } else {
- result = collapse_scan_pmd(mm, vma, addr, &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 = try_collapse_pte_mapped_thp(mm, addr, true);
- mmap_read_unlock(mm);
- goto handle_result;
/* Whitelisted set of results where continuing OK */
case SCAN_NO_PTE_TABLE:
+ case SCAN_PTE_MAPPED_HUGEPAGE:
case SCAN_PTE_NON_PRESENT:
case SCAN_PTE_UFFD_WP:
case SCAN_LACK_REFERENCED_PAGE:
--
2.53.0