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b=tCkxg9GYdvxn46hvMXLC79GUzwMDbakywUb/11REOxTOn8YwIkeu7ZPxitwXO6RbHvMWTi4Yot7jCvhOt8gfnustDU5aq7p3Xf1rE+IImwYuCzyRGv7a3HLRnJq7z6zKxylJCDZ/SJOV6fgB+WNxRDM8enwRfEO6yCdL44ZOPmT111IIWoamDbI88IcJxBvNrABznRQNrUwUHcIrE1sAOR/yIhFblEHjly05scFIMWh5GcFAbOjXESrZO1daD1N5SmBgYweilJa6H/m1TG5AgR0RRwaIqmnx3bCpprhtWkMZ+fVKuzBP1rN/ytkokoNyTJ5bzI4RPU+FyzVjc++kmw== Received: from DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) by BL1PR12MB5971.namprd12.prod.outlook.com (2603:10b6:208:39a::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8678.33; Wed, 30 Apr 2025 20:15:16 +0000 Received: from DS7PR12MB9473.namprd12.prod.outlook.com ([fe80::5189:ecec:d84a:133a]) by DS7PR12MB9473.namprd12.prod.outlook.com ([fe80::5189:ecec:d84a:133a%5]) with mapi id 15.20.8678.028; Wed, 30 Apr 2025 20:15:16 +0000 From: Zi Yan To: Nico Pache Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, akpm@linux-foundation.org, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, david@redhat.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, willy@infradead.org, peterx@redhat.com, shuah@kernel.org, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, dev.jain@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, Bagas Sanjaya Subject: Re: [PATCH v5 2/4] mm: document (m)THP defer usage Date: Wed, 30 Apr 2025 16:15:12 -0400 X-Mailer: MailMate (2.0r6249) Message-ID: In-Reply-To: <20250428182904.93989-3-npache@redhat.com> References: <20250428182904.93989-1-npache@redhat.com> <20250428182904.93989-3-npache@redhat.com> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BLAPR03CA0171.namprd03.prod.outlook.com (2603:10b6:208:32f::31) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|BL1PR12MB5971:EE_ X-MS-Office365-Filtering-Correlation-Id: ed59946f-59ed-4ec6-7911-08dd8823b899 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024|7053199007; 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Document its usage in the transhuge admin-guide. > > Reviewed-by: Bagas Sanjaya > Signed-off-by: Nico Pache > --- > Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------= > 1 file changed, 23 insertions(+), 8 deletions(-) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation= /admin-guide/mm/transhuge.rst > index 5c63fe51b3ad..c50253357793 100644 > --- a/Documentation/admin-guide/mm/transhuge.rst > +++ b/Documentation/admin-guide/mm/transhuge.rst > @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wi= de, application > may end up allocating more memory resources. An application may mmap a= > large region but only touch 1 byte of it, in that case a 2M page might= > be allocated instead of a 4k page for no good. This is why it's > -possible to disable hugepages system-wide and to only have them inside= > -MADV_HUGEPAGE madvise regions. > +possible to disable hugepages system-wide, only have them inside > +MADV_HUGEPAGE madvise regions, or defer them away from the page fault > +handler to khugepaged. > > Embedded systems should enable hugepages only inside madvise regions > to eliminate any risk of wasting any precious byte of memory and to > @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepa= ges and that don't > risk to lose memory by using hugepages, should use > madvise(MADV_HUGEPAGE) on their critical mmapped regions. > > +Applications that would like to benefit from THPs but would still like= a > +more memory conservative approach can choose 'defer'. This avoids > +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE= =2E > +Khugepaged will then scan the mappings for potential collapses into (m= )THP How about the text below? It explicitly states khugepaged behavior. Khugepaged will then scan all mappings, even those not explicitly marked with MADV_HUGEPAGE, for potential collapses into (m)THPs. > +pages. Admins using this the 'defer' setting should consider > +tweaking khugepaged/max_ptes_none. The current default of 511 may > +aggressively collapse your PTEs into PMDs. Lower this value to conserv= e > +more memory (i.e., max_ptes_none=3D64). > + > .. _thp_sysfs: > > sysfs > @@ -109,11 +119,14 @@ Global THP controls > > Transparent Hugepage Support for anonymous memory can be entirely disa= bled > (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE > -regions (to avoid the risk of consuming more memory resources) or enab= led > -system wide. This can be achieved per-supported-THP-size with one of::= > +regions (to avoid the risk of consuming more memory resources), deferr= ed to > +khugepaged, or enabled system wide. > + > +This can be achieved per-supported-THP-size with one of:: > > echo always >/sys/kernel/mm/transparent_hugepage/hugepages-kB/e= nabled > echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-kB/= enabled > + echo defer >/sys/kernel/mm/transparent_hugepage/hugepages-kB/en= abled > echo never >/sys/kernel/mm/transparent_hugepage/hugepages-kB/en= abled > > where is the hugepage size being addressed, the available sizes= > @@ -136,6 +149,7 @@ The top-level setting (for use with "inherit") can = be set by issuing > one of the following commands:: > > echo always >/sys/kernel/mm/transparent_hugepage/enabled > + echo defer >/sys/kernel/mm/transparent_hugepage/enabled > echo madvise >/sys/kernel/mm/transparent_hugepage/enabled > echo never >/sys/kernel/mm/transparent_hugepage/enabled > > @@ -286,7 +300,8 @@ of small pages into one large page:: > A higher value leads to use additional memory for programs. > A lower value leads to gain less thp performance. Value of > max_ptes_none can waste cpu time very little, you can > -ignore it. > +ignore it. Consider lowering this value when using > +``transparent_hugepage=3Ddefer`` > > ``max_ptes_swap`` specifies how many pages can be brought in from > swap when collapsing a group of pages into a transparent huge page:: > @@ -311,14 +326,14 @@ Boot parameters > > You can change the sysfs boot time default for the top-level "enabled"= > control by passing the parameter ``transparent_hugepage=3Dalways`` or > -``transparent_hugepage=3Dmadvise`` or ``transparent_hugepage=3Dnever``= to the > -kernel command line. > +``transparent_hugepage=3Dmadvise`` or ``transparent_hugepage=3Ddefer``= or > +``transparent_hugepage=3Dnever`` to the kernel command line. > > Alternatively, each supported anonymous THP size can be controlled by > passing ``thp_anon=3D[KMG],[KMG]:;[KMG]-[KMG]:``, > where ```` is the THP size (must be a power of 2 of PAGE_SIZE an= d > supported anonymous THP) and ```` is one of ``always``, ``madv= ise``, > -``never`` or ``inherit``. > +``defer``, ``never`` or ``inherit``. > > For example, the following will set 16K, 32K, 64K THP to ``always``, > set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M Otherwise, LGTM. Thanks. Reviewed-by: Zi Yan -- Best Regards, Yan, Zi