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

[Wei Yang]

On Sat, Feb 21, 2026 at 05:39:16PM +0800, Vernon Yang wrote:
>From: Vernon Yang <yanglincheng@xxxxxxxxxx>
>
>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@xxxxxxxxxx>
>Reviewed-by: Dev Jain <dev.jain@xxxxxxx>
>---
> 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;
>+	}

How about put cur_progress in struct collapse_control?

Then we don't need to check cur_progress every time before modification.

-- 
Wei Yang
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