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Message-ID: <20260201122554.1470071-3-vernon2gm@gmail.com>
Date: Sun, 1 Feb 2026 20:25:51 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: akpm@...ux-foundation.org,
david@...nel.org
Cc: lorenzo.stoakes@...cle.com,
ziy@...dia.com,
dev.jain@....com,
baohua@...nel.org,
lance.yang@...ux.dev,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Vernon Yang <yanglincheng@...inos.cn>
Subject: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number
From: Vernon Yang <yanglincheng@...inos.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
Signed-off-by: Vernon Yang <yanglincheng@...inos.cn>
---
mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
1 file changed, 31 insertions(+), 10 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d94b34e10bdf..df22b2274d92 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;
@@ -1240,7 +1243,8 @@ 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;
@@ -1256,13 +1260,18 @@ 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;
}
@@ -1396,6 +1405,12 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
result = SCAN_SUCCEED;
}
out_unmap:
+ if (cur_progress) {
+ if (_pte >= pte + HPAGE_PMD_NR)
+ *cur_progress = HPAGE_PMD_NR;
+ else
+ *cur_progress = _pte - pte + 1;
+ }
pte_unmap_unlock(pte, ptl);
if (result == SCAN_SUCCEED) {
result = collapse_huge_page(mm, start_addr, referenced,
@@ -2286,8 +2301,9 @@ 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;
@@ -2376,6 +2392,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
cond_resched_rcu();
}
}
+ if (cur_progress)
+ *cur_progress = max(xas.xa_index - start, 1UL);
rcu_read_unlock();
if (result == SCAN_SUCCEED) {
@@ -2455,6 +2473,7 @@ 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)))
@@ -2471,7 +2490,8 @@ 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);
@@ -2485,7 +2505,8 @@ 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)
@@ -2493,7 +2514,7 @@ 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
@@ -2816,7 +2837,7 @@ 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)) {
@@ -2831,7 +2852,7 @@ 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
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