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Thu, 26 Feb 2026 03:24:19 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.64.173]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 5615619560B6; Thu, 26 Feb 2026 03:24:09 +0000 (UTC) From: Nico Pache To: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, npache@redhat.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: [PATCH mm-unstable v15 04/13] mm/khugepaged: introduce collapse_max_ptes_none helper function Date: Wed, 25 Feb 2026 20:24:07 -0700 Message-ID: <20260226032407.233121-1-npache@redhat.com> In-Reply-To: <20260226031741.230674-1-npache@redhat.com> References: <20260226031741.230674-1-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: JTaxnkF4bW9wbFjKp9i4sM_KGWCFRjhupnXN-uT2DT8_1772076259 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true X-Stat-Signature: waejesj9gj5gzo5i5g1br3ih8yhxrbw3 X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3712D1C0002 X-HE-Tag: 1772076270-447458 X-HE-Meta: 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 unpSzQfu nxGyQJHm6GIWnvc+eP6D2hLsXVLKAT4rrIjeg025CXhZ3PGrCgYlcxix8wEHBNGznQTLlyOc1WY1/dFwbaHYrr5nJyIMkVouAhw5SlEZXw90GtIcKy7wpynnzsubEPJQRCQ6I7pxaIWM5OSprLdVkW0HNtIXzvVUhLfoy2qi4U5A9yx24dDaG1jyejElix1FP0JxS8IIlcsbeyy2W0HQFd/eXTb4CgHWJUBPYePX1Uvou+hJbplPMftizGKo3zSU4/0PeBdfg4nFNxH7Y6whxtY5iPI8Or8U9xAoOYx/bkThi/v6hYg1bN2m6Kl82ocLK72THTFJYaXdZnmef+/qECn+V3xf2Jp49ELCxdqFStxe4Meph5j+q1POniwgHM6uQ8oK9XnIjR0mQk/e4+d3NJL40AmTGBweQQCq/V/7H/ZM7z72lQ4zlt+uGHzsenj1rF0pMnkvQZ3nn9ZXakRL0ZrH6TQ== 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. Reviewed-by: Lorenzo Stoakes Reviewed-by: Baolin Wang Signed-off-by: Nico Pache --- mm/khugepaged.c | 40 +++++++++++++++++++++++++++++++++++++++- 1 file changed, 39 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index ecdbbf6a01a6..99f78f0e44c6 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -456,6 +456,36 @@ 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 + * + * 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 (COLLAPSE_MAX_PTES_LIMIT). 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) +{ + 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 == COLLAPSE_MAX_PTES_LIMIT) + return (1 << order) - 1; + + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %u\n", + COLLAPSE_MAX_PTES_LIMIT); + return -EINVAL; +} + void khugepaged_enter_vma(struct vm_area_struct *vma, vm_flags_t vm_flags) { @@ -541,10 +571,18 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, struct folio *folio = NULL; unsigned long addr = start_addr; pte_t *_pte; + int max_ptes_none; 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); + + if (cc->is_khugepaged) + max_ptes_none = collapse_max_ptes_none(order); + else + max_ptes_none = COLLAPSE_MAX_PTES_LIMIT; + + if (max_ptes_none == -EINVAL) + return result; for (_pte = pte; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) { -- 2.53.0