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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default Date: Fri, 05 Sep 2025 10:37:00 -0400 X-Mailer: MailMate (2.0r6272) Message-ID: <7B0B1E09-5083-449F-851D-FD63D32D2B3D@nvidia.com> In-Reply-To: <20250905141137.3529867-1-david@redhat.com> References: <20250905141137.3529867-1-david@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MN2PR22CA0019.namprd22.prod.outlook.com (2603:10b6:208:238::24) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|MW4PR12MB5625:EE_ X-MS-Office365-Filtering-Correlation-Id: f0c57d8a-9179-4cbd-ba5d-08ddec89addf X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024; 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Hm, not sure. Probably not urgent. > > Note that, as default, the THP shrinker is active > (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all > THPs are added to the deferred split lists. However, with the > max_ptes_none default we would never scan them. We would not do that. If > that's not desirable, we should just disable the shrinker as default, > also not adding all THPs to the deferred split lists. > > Easy to reproduce: > > 1) Allocate some THPs filled with 0s > > > #include > #include > #include > #include > #include > > const size_t size = 1024*1024*1024; > > int main(void) > { > size_t offs; > char *area; > > area = mmap(0, size, PROT_READ | PROT_WRITE, > MAP_ANON | MAP_PRIVATE, -1, 0); > if (area == MAP_FAILED) { > printf("mmap failed\n"); > exit(-1); > } > madvise(area, size, MADV_HUGEPAGE); > > for (offs = 0; offs < size; offs += getpagesize()) > area[offs] = 0; > pause(); > } > <\prog.c> > > 2) Trigger the shrinker > > E.g., memory pressure through memhog > > 3) Observe that THPs are not getting reclaimed > > $ cat /proc/`pgrep prog`/smaps_rollup > > Would list ~1GiB of AnonHugePages. With this fix, they would get > reclaimed as expected. > > Fixes: dafff3f4c850 ("mm: split underused THPs") > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: Zi Yan > Cc: Baolin Wang > Cc: "Liam R. Howlett" > Cc: Nico Pache > Cc: Ryan Roberts > Cc: Dev Jain > Cc: Barry Song > Cc: Usama Arif > Signed-off-by: David Hildenbrand > --- > mm/huge_memory.c | 3 --- > 1 file changed, 3 deletions(-) > LGTM. Acked-by: Zi Yan I also notice that thp_underused() checks num_zero_pages directly against khugepaged_max_ptes_none. This means mTHPs will never be regarded as underused. A similar issue you are discussing in Nico’s khugepaged mTHP support. Maybe checks against these khugepaged_max* variables should be calculated based on nr_pages of a large folio, like making these variables a ratio in other discussion. Best Regards, Yan, Zi