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Mon, 01 Dec 2025 12:48:40 -0500 X-MC-Unique: Bpno6CU_M2GZO9HDt3xq0Q-1 X-Mimecast-MFC-AGG-ID: Bpno6CU_M2GZO9HDt3xq0Q_1764611315 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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 71C23180057A; Mon, 1 Dec 2025 17:48:35 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.66.60]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 6E9A8180087B; Mon, 1 Dec 2025 17:48:25 +0000 (UTC) From: Nico Pache To: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org Cc: david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kas@kernel.org, aarcange@redhat.com, raquini@redhat.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, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de Subject: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function Date: Mon, 1 Dec 2025 10:46:18 -0700 Message-ID: <20251201174627.23295-8-npache@redhat.com> In-Reply-To: <20251201174627.23295-1-npache@redhat.com> References: <20251201174627.23295-1-npache@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Rspamd-Server: rspam12 X-Rspam-User: X-Rspamd-Queue-Id: 4EE534000D X-Stat-Signature: kyojag8stgxzc1nfg8qniz8kcyfdi8q5 X-HE-Tag: 1764611326-143782 X-HE-Meta: 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 2OoWbn2/ jMRjLIuZ8SWaDxSYKu+fDBvIB2CmVRZ1itlwzP0owreAo/cgWUJjqHOFkdNlZL2iXgcZYeZnrP3OY9qrBivVDfB8RWvegjk7SVoIlPVKmKiRUdRWfufp0GpwYV0FVjXaUBOxf/2zI0tgVm9H2Ug1RrKDcfRp66/KAD9yAsaLeFs9ctmyZSJatZLbh37hi8vMRWDMHBG2BLv1SUhdB+x9gv348HHu9U00eZRddDrm0dcUqctnMRmQ9Q/pmXML55s7lxqRtFBR+/KTMSLEQVjx2uQlBeY+eflweoqrbzKMaRAN/e0ERh3vezb5+NSMGiHo74zegO87JNY/6ejosg4XuYtOdRaDoTc4qpbzMEz8rvkdIKYTQn2YOdpfQyPzQqD+2O9g0lqr+k6m45+Ur1x7tgW+l42wx8gpo8lQa0PjQYj1ZKnK7yVPGkXO9JnKhK9hldw/31hdluynoicM= 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: The current mechanism for determining mTHP collapse scales the khugepaged_max_ptes_none value based on the target order. This introduces an undesirable feedback loop, or "creep", when max_ptes_none is set to a value greater than HPAGE_PMD_NR / 2. With this configuration, a successful collapse to order N will populate enough pages to satisfy the collapse condition on order N+1 on the next scan. This leads to unnecessary work and memory churn. To fix this issue introduce a helper function that will limit mTHP collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. This effectively supports two modes: - max_ptes_none=0: never introduce new none-pages for mTHP collapse. - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest available mTHP order. This removes the possiblilty of "creep", while not modifying any uAPI expectations. A warning will be emitted if any non-supported max_ptes_none value is configured with mTHP enabled. The limits can be ignored by passing full_scan=true, this is useful for madvise_collapse (which ignores limits), or in the case of collapse_scan_pmd(), allows the full PMD to be scanned when mTHP collapse is available. Signed-off-by: Nico Pache --- mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 42 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 8dab49c53128..f425238d5d4f 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm) wake_up_interruptible(&khugepaged_wait); } +/** + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse + * @order: The folio order being collapsed to + * @full_scan: Whether this is a full scan (ignore limits) + * + * For madvise-triggered collapses (full_scan=true), all limits are bypassed + * and allow up to HPAGE_PMD_NR - 1 empty PTEs. + * + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured + * khugepaged_max_ptes_none value. + * + * For mTHP collapses, we currently only support khugepaged_max_pte_none values + * of 0 or (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP + * collapse will be attempted + * + * Return: Maximum number of empty PTEs allowed for the collapse operation + */ +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan) +{ + /* ignore max_ptes_none limits */ + if (full_scan) + return HPAGE_PMD_NR - 1; + + if (!is_mthp_order(order)) + return khugepaged_max_ptes_none; + + /* Zero/non-present collapse disabled. */ + if (!khugepaged_max_ptes_none) + return 0; + + if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) + return (1 << order) - 1; + + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n", + HPAGE_PMD_NR - 1); + return -EINVAL; +} + void khugepaged_enter_vma(struct vm_area_struct *vma, vm_flags_t vm_flags) { @@ -550,7 +588,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, pte_t *_pte; int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0; const unsigned long nr_pages = 1UL << order; - int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order); + int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged); + + if (max_ptes_none == -EINVAL) + goto out; for (_pte = pte; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) { -- 2.51.1