[PATCH mm-new v8 2/4] mm: khugepaged: refine scan progress number

Vernon Yang
2026-02-21T09:39:59Z

From: Vernon Yang <yanglincheng@kylinos.cn>

Currently, each scan always increases "progress" by HPAGE_PMD_NR,
even if only scanning a single PTE/PMD entry.

- When only scanning a sigle PTE entry, 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.

- When the memory has been collapsed to PMD, let me provide a detailed
example:

The following data is traced by bpftrace on a desktop system. After
the system has been left idle for 10 minutes upon booting, a lot of
SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan
by khugepaged.

From trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file, the
following statuses were observed, with frequency mentioned next to them:

SCAN_SUCCEED : 1
SCAN_EXCEED_SHARED_PTE: 2
SCAN_PMD_MAPPED : 142
SCAN_NO_PTE_TABLE : 178
total progress size : 674 MB
Total time : 419 seconds, include khugepaged_scan_sleep_millisecs

The khugepaged_scan list save all task that support collapse into hugepage,
as long as the task is not destroyed, khugepaged will not remove it from
the khugepaged_scan list. This exist a phenomenon where task has already
collapsed all memory regions into hugepage, but khugepaged continues to
scan it, which wastes CPU time and invalid, and due to
khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
scanning a large number of invalid task, so scanning really valid task
is later.

After applying this patch, when the memory is either SCAN_PMD_MAPPED or
SCAN_NO_PTE_TABLE, just skip it, as follow:

SCAN_EXCEED_SHARED_PTE: 2
SCAN_PMD_MAPPED : 147
SCAN_NO_PTE_TABLE : 173
total progress size : 45 MB
Total time : 20 seconds

SCAN_PTE_MAPPED_HUGEPAGE is the same, for detailed data, refer to
https://lore.kernel.org/linux-mm/4qdu7owpmxfh3ugsue775fxarw5g2gcggbxdf5psj75nnu7z2u@cv2uu2yocaxq

Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
Reviewed-by: Dev Jain <dev.jain@arm.com>
---
mm/khugepaged.c | 42 ++++++++++++++++++++++++++++++++----------
1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e2f6b68a0011..61e25cf5424b 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;
@@ -1231,7 +1234,8 @@ 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,
+ struct vm_area_struct *vma, unsigned long start_addr,
+ bool *mmap_locked, unsigned int *cur_progress,
struct collapse_control *cc)
{
pmd_t *pmd;
@@ -1247,19 +1251,27 @@ 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 = 1;
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 = 1;
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;
@@ -2279,8 +2291,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
return result;
}

-static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
- struct file *file, pgoff_t start, struct collapse_control *cc)
+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;
struct address_space *mapping = file->f_mapping;
@@ -2370,6 +2383,12 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
}
}
rcu_read_unlock();
+ if (cur_progress) {
+ if (result == SCAN_PTE_MAPPED_HUGEPAGE)
+ *cur_progress = 1;
+ else
+ *cur_progress = HPAGE_PMD_NR;
+ }

if (result == SCAN_SUCCEED) {
if (cc->is_khugepaged &&
@@ -2448,6 +2467,7 @@ 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)))
@@ -2464,7 +2484,8 @@ 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);
@@ -2478,7 +2499,8 @@ 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)
@@ -2486,7 +2508,7 @@ 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
@@ -2809,7 +2831,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
mmap_locked = false;
*lock_dropped = true;
result = hpage_collapse_scan_file(mm, addr, file, pgoff,
- cc);
+ NULL, cc);

if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
mapping_can_writeback(file->f_mapping)) {
@@ -2824,7 +2846,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
fput(file);
} else {
result = hpage_collapse_scan_pmd(mm, vma, addr,
- &mmap_locked, cc);
+ &mmap_locked, NULL, cc);
}
if (!mmap_locked)
*lock_dropped = true;
--
2.51.0