Re: [PATCH v3 5/6] mm: khugepaged: skip lazy-free folios at scanning

[Lance Yang]

On 2026/1/4 13:41, Vernon Yang wrote:
> For example, create three task: hot1 -> cold -> hot2. After all three
> task are created, each allocate memory 128MB. the hot1/hot2 task
> continuously access 128 MB memory, while the cold task only accesses
> its memory briefly andthen call madvise(MADV_FREE). However, khugepaged
> still prioritizes scanning the cold task and only scans the hot2 task
> after completing the scan of the cold task.
>
> So if the user has explicitly informed us via MADV_FREE that this memory
> will be freed, it is appropriate for khugepaged to skip it only, thereby
> avoiding unnecessary scan and collapse operations to reducing CPU
> wastage.
>
> Here are the performance test results:
> (Throughput bigger is better, other smaller is better)
>
> Testing on x86_64 machine:
>
> | task hot2           | without patch | with patch    |  delta  |
> |---------------------|---------------|---------------|---------|
> | total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  |
> | cycles per access   |  4.96         |  2.21         | -55.44% |
> | Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  |
> | dTLB-load-misses    |  284814532    |  69597236     | -75.56% |
>
> Testing on qemu-system-x86_64 -enable-kvm:
>
> | task hot2           | without patch | with patch    |  delta  |
> |---------------------|---------------|---------------|---------|
> | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
> | cycles per access   |  7.29         |  2.07         | -71.60% |
> | Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% |
> | dTLB-load-misses    |  241600871    |  3216108      | -98.67% |
>
> Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
> ---
>   include/trace/events/huge_memory.h | 1 +
>   mm/khugepaged.c                    | 6 ++++++
>   2 files changed, 7 insertions(+)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 01225dd27ad5..e99d5f71f2a4 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -25,6 +25,7 @@
>   	EM( SCAN_PAGE_LRU,		"page_not_in_lru")		\
>   	EM( SCAN_PAGE_LOCK,		"page_locked")			\
>   	EM( SCAN_PAGE_ANON,		"page_not_anon")		\
> +	EM( SCAN_PAGE_LAZYFREE,		"page_lazyfree")		\
>   	EM( SCAN_PAGE_COMPOUND,		"page_compound")		\
>   	EM( SCAN_ANY_PROCESS,		"no_process_for_page")		\
>   	EM( SCAN_VMA_NULL,		"vma_null")			\
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 30786c706c4a..1ca034a5f653 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -45,6 +45,7 @@ enum scan_result {
>   	SCAN_PAGE_LRU,
>   	SCAN_PAGE_LOCK,
>   	SCAN_PAGE_ANON,
> +	SCAN_PAGE_LAZYFREE,
>   	SCAN_PAGE_COMPOUND,
>   	SCAN_ANY_PROCESS,
>   	SCAN_VMA_NULL,
> @@ -1337,6 +1338,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>   		}
>   		folio = page_folio(page);
>   
> +		if (folio_is_lazyfree(folio)) {
> +			result = SCAN_PAGE_LAZYFREE;
> +			goto out_unmap;
> +		}

That's a bit tricky ... I don't think we need to handle MADV_FREE pages
differently :)

MADV_FREE pages are likely cold memory, but what if there are just
a few MADV_FREE pages in a hot memory region? Skipping the entire
region would be unfortunate ...

Also, even if we skip these pages now, after they are reclaimed, they
become pte_none. Then khugepaged will try to collapse them anyway
(based on khugepaged_max_ptes_none). So skipping them just delays
things, it does not really change the final result ;)

Thanks,
Lance

> +
>   		if (!folio_test_anon(folio)) {
>   			result = SCAN_PAGE_ANON;
>   			goto out_unmap;