Re: [PATCH mm-unstable v1 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()
From: Lance Yang
Date: Sat Feb 14 2026 - 03:24:48 EST
On 2026/2/13 04:26, Nico Pache wrote:
On Thu, Feb 12, 2026 at 1:04 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
On 2/12/26 03:25, Nico Pache wrote:
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properly indicates that this process is no longer valid to operate on.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: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
mm/khugepaged.c | 121 ++++++++++++++++++++++++++----------------------
1 file changed, 66 insertions(+), 55 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fa41480f6948..0839a781bedd 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2395,6 +2395,62 @@ 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);
+
+ if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
+ 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);
+ }
+ 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 +2522,9 @@ 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 +2830,7 @@ 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) {
@@ -2809,46 +2841,25 @@ 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:
+ if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
+ triggered_wb = true;
+ goto retry;
+ }
Having triggered_wb set where writeback is not actually triggered is
suboptimal.
Good catch!
It took me a second to figure out what you were referring to, but I
see it now. if we return SCAN_PAGE_D_OR_WB but the can_writeback fails
it still retries.
Would be an appropriate solution if can_writeback fails to modify the
return value.
ie)
Yep, we're on the right track. IIRC, David's concern has two parts:
1) Avoid retry when writeback wasn't actually triggered
(mapping_can_writeback() fails)
2) Avoid calling filemap_write_and_wait_range() twice on retry
The proposed approach below addresses #1, but we still need to tackle #2.
The issue is that on the retry, collapse_single_pmd() doesn't know that
writeback was already performed in the previous round, so it could call
filemap_write_and_wait_range() again if the page is stil dirty.
if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK) {
if (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);
} else {
result = SCAN_(SOMETHING?)
}
}
fput(file);
we dont have a enum that fits this description but we want one that
will continue.
Cheers!
-- Nico
And you can tell that by realizing that you would now retry once even
thought the mapping does not support writeback (
mapping_can_writeback(file->f_mapping)) and no writeback actually happened.
Further, we would also try to call filemap_write_and_wait_range() now
twice instead of only during the first round.
Right. Let's avoid calling it twice.
Cheers,
Lance