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Message-ID: <56c004dd-fe54-42a7-a8a0-38aeaf97c8c4@linux.dev>
Date: Wed, 14 Jan 2026 20:44:52 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
Vernon Yang <vernon2gm@...il.com>
Cc: lorenzo.stoakes@...cle.com, ziy@...dia.com, dev.jain@....com,
baohua@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Vernon Yang <yanglincheng@...inos.cn>, akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new v4 5/6] mm: khugepaged: skip lazy-free folios at
scanning
On 2026/1/14 19:50, David Hildenbrand (Red Hat) wrote:
> On 1/11/26 13:19, 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 | 17 +++++++++++++++++
>> 2 files changed, 18 insertions(+)
>>
>> diff --git a/include/trace/events/huge_memory.h b/include/trace/
>> events/huge_memory.h
>> index 3d1069c3f0c5..e3856f8ab9eb 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 6df2857d94c6..8a7008760566 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,
>> @@ -1258,6 +1259,7 @@ static enum scan_result
>> hpage_collapse_scan_pmd(struct mm_struct *mm,
>> pmd_t *pmd;
>> pte_t *pte, *_pte;
>> int none_or_zero = 0, shared = 0, referenced = 0;
>> + int lazyfree = 0;
>> enum scan_result result = SCAN_FAIL;
>> struct page *page = NULL;
>> struct folio *folio = NULL;
>> @@ -1343,6 +1345,21 @@ static enum scan_result
>> hpage_collapse_scan_pmd(struct mm_struct *mm,
>> }
>> folio = page_folio(page);
>> + if (cc->is_khugepaged && !pte_dirty(pteval) &&
>> + folio_is_lazyfree(folio)) {
>> + ++lazyfree;
>> +
>> + /*
>> + * The lazyfree folios are reclaimed and become pte_none.
>> + * Ensure they do not continue to be collapsed when
>> + * skipped ahead.
>> + */
>> + if ((lazyfree + none_or_zero) > khugepaged_max_ptes_none) {
>> + result = SCAN_PAGE_LAZYFREE;
>> + goto out_unmap;
>
> I dislike adding another khugepaged_max_ptes_none check. Gah.
>
>
> Can't we should just keep it simple and do
>
> if (!pte_dirty(pteval) && folio_is_lazyfree(folio)) {
> result = SCAN_PAGE_LAZYFREE;
> goto out_unmap;
> }
>
> Reasoning: once they are none, we have a zero-filled page that e.g., the
> deferred shrinker can reclaim.
>
> If you collapse with a lazyfree page, that content will never be none
> and the deferred shrinker cannot reclaim them.
>
> So there is a real difference between them being none and them still
> being around.
>
>
> We could also try turning them into none entries here, that is, test of
> we can discard them, to then just threat them like none entries.
Right, I would prefer turning them into none entries, but that seems to
complicate things a bit, e.g., making sure we don't copy content from them
during collapse ...
So let's keep it simple: just bail out if the page is lazyfree and clean :)
>
>
> Why don't we want to similarly handle this in
> __collapse_huge_page_isolate() ?
Yeah, that should be added there as well.
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