Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

David Hildenbrand (Arm)
2026-03-31T21:04:03Z

On 3/31/26 22:50, David Hildenbrand (Arm) wrote:
> On 3/31/26 22:06, Lorenzo Stoakes (Oracle) wrote:
>> On Tue, Mar 31, 2026 at 10:00:58PM +0200, David Hildenbrand (Arm) wrote:
>>>
>>> Right.
>>>
>>> The original code used
>>>
>>> bool mmap_locked = true;
>>>
>>> If the fix get squashed, maybe we should revert to that handling to
>>> minimize the churn?
>>
>> Well collapse_single_pmd() and all descendents would need to be updated to
>> invert the boolean, not sure it's worth it.
>
> The code is confusing me.
>
> In the old code, collapse_single_pmd() was called with "mmap_locked ==
> true" and set "mmap_locked=false".
>
> Now we call it with "mmap_unlocked=false" and it sets "mmap_unlocked=true".
>
> So far so good.
>
> However, collapse_scan_pmd() consumed "mmap_locked", now it effectively
> consumes "mmap_unlocked".

Okay, I'm too confused for today and give up :)

FWIW, this is the effective change, and the effective changes to
mmap_unlocked and lock_dropped ... confuse me:

mm/khugepaged.c | 146 +++++++++++++++++++++++++-----------------------
1 file changed, 76 insertions(+), 70 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 26c38934e829..3ffff9f98b2f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1253,7 +1253,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a

static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long start_addr,
- bool *mmap_locked, struct collapse_control *cc)
+ bool *lock_dropped, struct collapse_control *cc)
{
pmd_t *pmd;
pte_t *pte, *_pte;
@@ -1428,7 +1428,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
result = collapse_huge_page(mm, start_addr, referenced,
unmapped, cc);
/* collapse_huge_page will return with the mmap_lock released */
- *mmap_locked = false;
+ *lock_dropped = true;
}
out:
trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
@@ -2420,6 +2420,67 @@ 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 *lock_dropped,
+ 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;
+
+ mmap_assert_locked(mm);
+
+ if (vma_is_anonymous(vma)) {
+ result = collapse_scan_pmd(mm, vma, addr, lock_dropped, cc);
+ goto end;
+ }
+
+ file = get_file(vma->vm_file);
+ pgoff = linear_page_index(vma, addr);
+
+ mmap_read_unlock(mm);
+ *lock_dropped = true;
+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)
@@ -2481,46 +2542,21 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);

while (khugepaged_scan.address < hend) {
- bool mmap_locked = true;
+ bool lock_dropped = false;

cond_resched();
if (unlikely(collapse_test_exit_or_disable(mm)))
goto breakouterloop;

- VM_BUG_ON(khugepaged_scan.address < hstart ||
+ VM_WARN_ON_ONCE(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, &lock_dropped, cc);
/* move to next address */
khugepaged_scan.address += HPAGE_PMD_SIZE;
- if (!mmap_locked)
+ if (lock_dropped)
/*
* We released mmap_lock so break loop. Note
* that we drop mmap_lock before all hugepage
@@ -2795,7 +2831,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
unsigned long hstart, hend, addr;
enum scan_result last_fail = SCAN_FAIL;
int thps = 0;
- bool mmap_locked = true;
+ bool mmap_unlocked = false;

BUG_ON(vma->vm_start > start);
BUG_ON(vma->vm_end < end);
@@ -2817,13 +2853,12 @@ 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) {
+ if (mmap_unlocked) {
cond_resched();
mmap_read_lock(mm);
- mmap_locked = true;
+ mmap_unlocked = false;
+ *lock_dropped = true;
result = hugepage_vma_revalidate(mm, addr, false, &vma,
cc);
if (result != SCAN_SUCCEED) {
@@ -2833,45 +2868,14 @@ 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_unlocked, 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_NON_PRESENT:
@@ -2894,8 +2898,10 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,

out_maybelock:
/* Caller expects us to hold mmap_lock on return */
- if (!mmap_locked)
+ if (mmap_unlocked) {
+ *lock_dropped = false;
mmap_read_lock(mm);
+ }
out_nolock:
mmap_assert_locked(mm);
mmdrop(mm);
--
2.43.0

--
Cheers,

David