Re: [PATCH mm-new v6 4/5] mm: khugepaged: skip lazy-free folios

[Vernon Yang <vernon2gm@gmail.com>]

On Thu, Feb 5, 2026 at 5:24 AM David Hildenbrand (arm) <david@kernel.org> wrote:
>
> On 2/1/26 13:25, 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                    | 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 df22b2274d92..b4def001ccd0 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)) {
>
> Should be aligned as
>
>                 if (cc->is_khugepaged && !pte_dirty(pteval) &&
>                     folio_test_lazyfree(folio)) {

LGTM, Thank you for review and suggestion, I will do it in the next version.

> But you could just have it in a single line.

If it is placed on a single line, it will exceed 80 characters.

> > +                     result = SCAN_PAGE_LAZYFREE;
> > +                     goto out;
> > +             }
> > +
> >               /* See hpage_collapse_scan_pmd(). */
> >               if (folio_maybe_mapped_shared(folio)) {
> >                       ++shared;
> > @@ -1332,6 +1339,12 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >               }
> >               folio = page_folio(page);
> >
> > +             if (cc->is_khugepaged && !pte_dirty(pteval) &&
> > +                             folio_test_lazyfree(folio)) {
> > +                     result = SCAN_PAGE_LAZYFREE;
> > +                     goto out_unmap;
> > +             }
>
> Dito.
>
> > +
> >               if (!folio_test_anon(folio)) {
> >                       result = SCAN_PAGE_ANON;
> >                       goto out_unmap;
>
> Surprised that there is no need to add checks for SCAN_PAGE_LAZYFREE
> anywhere, but it's similar to SCAN_PAGE_LOCK just that we cannot ever
> run into it for madvise.
>
> Acked-by: David Hildenbrand (arm) <david@kernel.org>

Thank you for review and explanation.

> --
> Cheers,
>
> David