Re: [PATCH mm-new v5 4/5] mm: khugepaged: skip lazy-free folios

[Lance Yang <lance.yang@linux.dev>]

On 2026/1/23 16:22, Vernon Yang wrote:
> From: Vernon Yang <yanglincheng@kylinos.cn>
> 
> 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.
> 
> And if we collapse with a lazyfree page, that content will never be none
> and the deferred shrinker cannot reclaim them.
> 
> 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@kylinos.cn>
> ---
>   include/trace/events/huge_memory.h |  1 +
>   mm/khugepaged.c                    | 11 +++++++++++
>   2 files changed, 12 insertions(+)
> 
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 384e29f6bef0..bcdc57eea270 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 de95029e3763..be1c09842ea2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -46,6 +46,7 @@ enum scan_result {
>   	SCAN_PAGE_LRU,
>   	SCAN_PAGE_LOCK,
>   	SCAN_PAGE_ANON,
> +	SCAN_PAGE_LAZYFREE,
>   	SCAN_PAGE_COMPOUND,
>   	SCAN_ANY_PROCESS,
>   	SCAN_VMA_NULL,
> @@ -583,6 +584,11 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>   		folio = page_folio(page);
>   		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
>   
> +		if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {

I'm wondering if we need "cc->is_khugepaged &&" as well here?

We should allow users to enforce collapse via the madvise_collapse()
path even if pages are marked lazyfree, IMHO.

> +			result = SCAN_PAGE_LAZYFREE;
> +			goto out;
> +		}
> +
>   		/* See hpage_collapse_scan_pmd(). */
>   		if (folio_maybe_mapped_shared(folio)) {
>   			++shared;
> @@ -1330,6 +1336,11 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>   		}
>   		folio = page_folio(page);
>   
> +		if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {

Ditto.

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