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

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

On 2026/1/5 09:48, Vernon Yang wrote:
> On Sun, Jan 04, 2026 at 08:10:17PM +0800, Lance Yang wrote:
>>
>>
>> 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@kylinos.cn>
>>> ---
>>>    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 ...
> 
> If there are hot in lazyfree folios, the folio will be set as non-lazyfree
> in the memory reclaim path, it is not skipped in the next scan in the
> khugepaged.
> 
> shrink_folio_list()
>    try_to_unmap()
>      folio_set_swapbacked()
> 
> If there are no hot in lazyfree folios, continuing the collapse would
> waste CPU and require a long wait (khugepaged_scan_sleep_millisecs).
> Additionally, due to collapse hugepage become non-lazyfree, preventing
> the rapid release of lazyfree folios in the memory reclaim path.
> 
> So skipping lazy-free folios make sense here for us.
> 
> If I missed something, please let me know, thank!

I'm not saying lazyfree pages become hot :)

If a PMD region has mostly hot pages but just a few lazyfree
pages, we would skip the entire region. Those hot pages won't
be collapsed.

> 
>> 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 ;)
> 
> This patch just resolve scene for hot1 -> cold -> hot2.
> 
> --
> Thanks,
> Vernon