From nobody Mon Feb 9 05:59:20 2026 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0E4BE1D5CDE for ; Tue, 11 Feb 2025 00:41:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739234521; cv=none; b=WdXaT+4Qp59xsGb8yKaKJxwcTE1fAHZJ14eYQKsHnIJBW3pyLbxpKNP8WW2tfpQtOJ1ChkDQp9W+ZZ78jsCrnmkZqKJAN6DK/KDUcUpHPXlI0Z9/MqVXZG0ba+c3HCsIGKSBfOPI1diINp5TT+APkC1HKM+Y6+fwhTlejl15PnE= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739234521; c=relaxed/simple; bh=5u2bXN6MfJZJYcCxg9TCJ8hq53od4+g/Fj1IvtxnxiI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=lSpWvqH6xKGwTpAgAbX4CMtV280kdKPXZq14T99f6u/++rxxGhB/Vq0lSnzWC8UlSNUdiJvIlK7r2I6ofgoj5x7sZOh1lj6J3Jg92qF/mcWh3eaWxLAPzZQ+9VmBqtYBz7RoejAD8lbzmfk6ofzPagI0KHYntsY53Bb9Sz2E1X4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=HIN5YwFp; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HIN5YwFp" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1739234519; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=WeJh2P4bxed8LMp5Ek4gDvhHkD3M/Aeurz6cvFHf2wo=; b=HIN5YwFpWhgZRXew4bTp8UgGJG8VoW4bO9uHD3qR8qTyDlfh14tGXAFSx2+WeBKiOH+mJ/ 9+k63q7awbseEp+KN7GfvgyUy97v3DlUHqCbWyT1NTgv0k5Qt4yab+WyeI7hTe6TPSJlKf FnBAuxh4Va+Z1U1G4FE8dnGLVsE9KPk= Received: from mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-161-QP1PNo-sPkebWvclG8DInw-1; Mon, 10 Feb 2025 19:41:55 -0500 X-MC-Unique: QP1PNo-sPkebWvclG8DInw-1 X-Mimecast-MFC-AGG-ID: QP1PNo-sPkebWvclG8DInw Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B53FB1956086; Tue, 11 Feb 2025 00:41:50 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.88.129]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 7117518004A7; Tue, 11 Feb 2025 00:41:40 +0000 (UTC) From: Nico Pache To: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, jack@suse.cz, srivatsa@csail.mit.edu, haowenchao22@gmail.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ziy@nvidia.com, jglisse@google.com, surenb@google.com, vishal.moola@gmail.com, zokeefe@google.com, zhengqi.arch@bytedance.com, jhubbard@nvidia.com, 21cnbao@gmail.com, willy@infradead.org, kirill.shutemov@linux.intel.com, david@redhat.com, aarcange@redhat.com, raquini@redhat.com, dev.jain@arm.com, sunnanyong@huawei.com, usamaarif642@gmail.com, audra@redhat.com, akpm@linux-foundation.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, tiwai@suse.de, baolin.wang@linux.alibaba.com, corbet@lwn.net, shuah@kernel.org Subject: [RFC v2 2/5] mm: document transparent_hugepage=defer usage Date: Mon, 10 Feb 2025 17:40:51 -0700 Message-ID: <20250211004054.222931-3-npache@redhat.com> In-Reply-To: <20250211004054.222931-1-npache@redhat.com> References: <20250211004054.222931-1-npache@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Content-Type: text/plain; charset="utf-8" The new transparent_hugepage=3Ddefer option allows for a more conservative approach to THPs. Document its usage in the transhuge admin-guide. Signed-off-by: Nico Pache --- Documentation/admin-guide/mm/transhuge.rst | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/adm= in-guide/mm/transhuge.rst index dff8d5985f0f..b3b18573bbb4 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 wide, = 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. =20 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 hugepages = and that don't risk to lose memory by using hugepages, should use madvise(MADV_HUGEPAGE) on their critical mmapped regions. =20 +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. +Khugepaged will then scan the mappings for potential collapses into PMD +sized 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 conserve +more memory (ie. max_ptes_none=3D64). + .. _thp_sysfs: =20 sysfs @@ -136,6 +146,7 @@ The top-level setting (for use with "inherit") can be s= et by issuing one of the following commands:: =20 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 =20 @@ -274,7 +285,8 @@ 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`` =20 ``max_ptes_swap`` specifies how many pages can be brought in from swap when collapsing a group of pages into a transparent huge page:: @@ -299,8 +311,8 @@ Boot parameters =20 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. =20 Alternatively, each supported anonymous THP size can be controlled by passing ``thp_anon=3D[KMG],[KMG]:;[KMG]-[KM= G]:``, --=20 2.48.1