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

[Vernon Yang <vernon2gm@gmail.com>]

On Mon, Jan 5, 2026 at 10:51 AM Lance Yang <lance.yang@linux.dev> wrote:
>
> 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.

Same above, the lazyfree folios will be set as non-lazyfree
in the memory reclaim path, it is not skipped in the next scan,
the PMD region will collapse :)

> >
> >> 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
>