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Sun, 04 Jan 2026 17:48:41 -0800 (PST) Date: Mon, 5 Jan 2026 09:48:32 +0800 From: Vernon Yang To: Lance Yang Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, richard.weiyang@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang , akpm@linux-foundation.org, david@kernel.org Subject: Re: [PATCH v3 5/6] mm: khugepaged: skip lazy-free folios at scanning Message-ID: <3lbptab7e2nhqilwnoccq6kxks2r55j3ffqtslt62o2qtgulk5@w4mwglb2kd75> References: <20260104054112.4541-1-yanglincheng@kylinos.cn> <20260104054112.4541-6-yanglincheng@kylinos.cn> <9c82ffaa-5f62-4110-80cc-00f0c46e90fb@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9c82ffaa-5f62-4110-80cc-00f0c46e90fb@linux.dev> X-Rspam-User: X-Rspamd-Queue-Id: 3D1291A0008 X-Rspamd-Server: rspam04 X-Stat-Signature: fzwgxadkf1jpf5gp7oqkaj63ie3sx1hw X-HE-Tag: 1767577723-791276 X-HE-Meta: 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 mnhjkuiS 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > --- > > 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