Re: [PATCH mm-new v7 2/5] mm: khugepaged: refine scan progress number

[Dev Jain <dev.jain@arm.com>]

On 07/02/26 1:46 pm, Vernon Yang wrote:
> 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
>
> Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> ---
>  mm/khugepaged.c | 38 ++++++++++++++++++++++++++++----------
>  1 file changed, 28 insertions(+), 10 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4049234e1c8b..8b68ae3bc2c5 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;
> @@ -1256,19 +1260,27 @@ 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;
> @@ -2288,8 +2300,9 @@ 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;
> @@ -2378,6 +2391,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>  			cond_resched_rcu();
>  		}
>  	}
> +	if (cur_progress)
> +		*cur_progress = HPAGE_PMD_NR;
>  	rcu_read_unlock();
>  
>  

Nit: Could move this at the end of the function. Looks weird before the
rcu_read_unlock.

Reviewed-by: Dev Jain <dev.jain@arm.com>