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Message-ID: <CACZaFFNJO+-yZdDPdN-5rQLuJvT5cp2pbtqV13FHo9PeqtxARg@mail.gmail.com>
Date: Mon, 5 Jan 2026 11:12:52 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: lorenzo.stoakes@...cle.com, ziy@...dia.com, dev.jain@....com,
baohua@...nel.org, richard.weiyang@...il.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>,
akpm@...ux-foundation.org, david@...nel.org
Subject: Re: [PATCH v3 5/6] mm: khugepaged: skip lazy-free folios at scanning
On Mon, Jan 5, 2026 at 10:51 AM Lance Yang <lance.yang@...ux.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@...inos.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
>
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