Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios

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

On 2026/2/8 05:38, David Hildenbrand (Arm) wrote:
> On 2/7/26 14:51, Lance Yang wrote:
>>
>>
>> On 2026/2/7 16:34, Barry Song wrote:
>>> On Sat, Feb 7, 2026 at 4:16 PM Vernon Yang <vernon2gm@gmail.com> 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 and then 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>
>>>> Acked-by: David Hildenbrand (arm) <david@kernel.org>
>>>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>>>> ---
>>>>   include/trace/events/huge_memory.h |  1 +
>>>>   mm/khugepaged.c                    | 13 +++++++++++++
>>>>   2 files changed, 14 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 8b68ae3bc2c5..0d160e612e16 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 +584,12 @@ 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 (cc->is_khugepaged && !pte_dirty(pteval) &&
>>>> +                   folio_test_lazyfree(folio)) {
>>>
>>> We have two corner cases here:
>>
>> Good catch!
>>
>>>
>>> 1. Even if a lazyfree folio is dirty, if the VMA has the VM_DROPPABLE 
>>> flag,
>>> a lazyfree folio may still be dropped, even when its PTE is dirty.
> 
> Good point!
> 
>>
>> Right. When the VMA has VM_DROPPABLE, we would drop the lazyfree folio
>> regardless of whether it (or the PTE) is dirty in try_to_unmap_one().
>>
>> So, IMHO, we could go with:
>>
>> cc->is_khugepaged && folio_test_lazyfree(folio) &&
>>      (!pte_dirty(pteval) || (vma->vm_flags & VM_DROPPABLE))
> 
> Hm. In a VM_DROPPABLE mapping all folios should be marked as lazy-free 
> (see folio_add_new_anon_rmap()).

Ah, I missed that apparently :)

> The new (collapse) folio will also be marked lazy (due to 
> folio_add_new_anon_rmap()) free and can just get dropped any time.
> 
> So likely we should just not skip collapse for lazyfree folios in 
> VM_DROPPABLE mappings?
> 
> if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
>      folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
>      ...
> }

Yep. That should be doing the trick. Thanks!