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[PATCH] mm: thp: default defrag mode to defer+madvise Cc: "Baolin Wang" , "Ryan Roberts" , "Barry Song" , , To: "Ferran Duarri" , "Andrew Morton" , "David Hildenbrand" , "Lorenzo Stoakes" , "Johannes Weiner" From: "Zi Yan" X-Mailer: aerc 0.22.0 References: <20260820190825.221308-1-ferran.duarri@me.com> In-Reply-To: <20260820190825.221308-1-ferran.duarri@me.com> X-ClientProxiedBy: BL1PR13CA0367.namprd13.prod.outlook.com (2603:10b6:208:2c0::12) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|PH0PR12MB7862:EE_ X-MS-Office365-Filtering-Correlation-Id: 42e8229d-e23b-4ccd-4883-08defef7c8ce X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|23010399003|1800799024|56012099006|5023799004|11063799006|10067099003|22082099003|18002099003|6133799003; X-Microsoft-Antispam-Message-Info: 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 YVz6CtZ5 EvcIktE36lAnRWjmBIHYecgXxA7tqfpYnBtdd0QL+G18yLmELkyvpyADFmb0qiXXmzswzqeCoizt2u8GgwwDb/DGWI/6BechGmc15Q3fw4eaTGTRNr2uDa++j1kBUT3mRvRvkHgoZk5CY8CVUlATabSOVkYGPLn8CTyjq1IQgShzVRe+W0JZt0Aql7V4HXJ4871zgMyLD7MyuqqLYatfpCQZ6d4s09IcrSHAN0DP5nmxuXr5LbQy29M4mKip4Uc68boSGIZlBWWQ7GaBU37+pY6N83UxrnkmAWfN3R7bBQ8pNWUlYKUtCKfyKkuaA2e8+H08F+TxqQM7Ws6Qxb1FWg1qK8QTGexqZftVfxRo9Hok54ZbVQgvfBnTwcpI2KuLh4BG8/6WMqVmS9Zto5lc2151vFA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu Aug 20, 2026 at 3:08 PM EDT, Ferran Duarri wrote: > transparent_hugepage_flags unconditionally sets > TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, so the built-in defrag mode > is always "madvise" regardless of which of CONFIG_TRANSPARENT_HUGEPAGE_ > ALWAYS or _MADVISE was selected. Those Kconfig options choose whether > THP is applied by default; they do not express a preference about how > hard the allocator should work to produce a huge page, yet the defrag > default is pinned as if they did. > > In "madvise" mode a fault on a MADV_HUGEPAGE region performs direct > compaction in the fault path. Paired with transparent_hugepage=3Dmadvise > that is a bounded cost, since only regions that asked for it are > affected. Paired with transparent_hugepage=3Dalways every anonymous fault > becomes eligible, and under memory pressure the faulting thread can > stall in compaction. > > "defer+madvise" keeps the same set of regions eligible for huge pages > and the same allocation attempt, but on failure it wakes kswapd and > khugepaged to compact in the background instead of doing it inline. The > fault proceeds with small pages and the region is collapsed later. This > is the configuration long recommended to users running THP=3Dalways with > large anonymous working sets, and it is a strictly weaker stall > guarantee to make the default. > > This changes only the compiled-in default; > /sys/kernel/mm/transparent_hugepage/defrag continues to accept every > existing mode, including the current "madvise" behaviour. > > No stall measurement is offered with this patch. On the machine that > prompted it the fault path never reaches direct compaction at all: > thp_fault_alloc is 60682 against thp_fault_fallback 0, and compact_stall > is 0, because memory has stayed abundant enough that no huge-page > allocation has had to fall back. That makes the box unable to testify > either way, and a number gathered under those conditions would measure > nothing. The argument above is a correctness one about which knob the > Kconfig choice is entitled to set, and it should be judged on that. > > Signed-off-by: Ferran Duarri > --- > mm/huge_memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index b118bcd392cb..749f669cca56 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -63,7 +63,7 @@ unsigned long transparent_hugepage_flags __read_mostly = =3D > #ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE > (1< #endif > - (1< + (1< (1< (1< =20 +Johannes, since he said Meta heavily rely khugepaged to make good use of THP, he might give more insight here. The patch adds additional kswapd/kcompactd work when a system fails to allocate THP for non madvised THP-eligible memory regions. If the system's CPU utilization is high, kswapd/kcompactd can take CPU resrouces from others, right? Performance numbers are needed to justify this change. --=20 Best Regards, Yan, Zi