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

[Vernon Yang <vernon2gm@gmail.com>]

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!

> 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