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Thu, 22 Jan 2026 19:31:13 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.88.59]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id C07CA1958DC1; Thu, 22 Jan 2026 19:31:03 +0000 (UTC) From: Nico Pache To: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: npache@redhat.com, akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, jannh@google.com, pfalcato@suse.de, jackmanb@google.com, hannes@cmpxchg.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, zokeefe@google.com, rientjes@google.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com Subject: [PATCH mm-unstable v14 07/16] khugepaged: introduce collapse_max_ptes_none helper function Date: Thu, 22 Jan 2026 12:28:32 -0700 Message-ID: <20260122192841.128719-8-npache@redhat.com> In-Reply-To: <20260122192841.128719-1-npache@redhat.com> References: <20260122192841.128719-1-npache@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: otGgw-lXILei5XVGQW0mKZYjcRocM4JXznKjM5qoyYA_1769110273 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true X-Rspamd-Queue-Id: DF0E540008 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: rg8itd4p4qxzib1zano9gxemfur86he8 X-HE-Tag: 1769110281-999991 X-HE-Meta: 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 WhT72E1Q 0rcmhyFj+wvFybW4TwKhfJ+P6AuEvGqzemAROwpFCafUrQYPH+yogA04rXaAlT+P5PxA7dt014GkzCJDNCGoeOin2mGp/I+BlVd8AnsLaIQJg0m3EOefqpBKwTy9ADcdojzxwcmgIjT9v2j8ogbSUAfRSIJ6GqMKvC0tsT86/3PYfiGTP9heGpSXhwUregTWAdbOaF2xtiAXuujt9m4TW9gFQ3r0qPIZSbOCsYGT/+77Ogr3/+avJLH6udhMl7UNFvbX9KJcOqSmZB7POes/Kmj6QvicwgfVN5ZR38FjaLqokplHPacVDVNuuyA4a4ox4ZP7qhex8FHqChFhACksJBNdGox3XAmOX4vhCJb/wP7pKqHhBqhiAqEyZdQvb6zV42p31c/zrQEvsmhI5Nyr3KunzEGsmdGH3NPScb636syP5UZi3mz43mXi0QSxl63fAVeY3nnNwgmJ+ryA5aaoDNWxJaY0iItopiXHI 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. Reviewed-by: Baolin Wang 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 0f68902edd9a..9b7e05827749 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -460,6 +460,44 @@ 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_pmd_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) { @@ -548,7 +586,10 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, int none_or_zero = 0, shared = 0, referenced = 0; enum scan_result result = SCAN_FAIL; 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) + return result; for (_pte = pte; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) { -- 2.52.0