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Wed, 22 Oct 2025 18:39:19 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.64.41]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 04CFA19560B4; Wed, 22 Oct 2025 18:39:09 +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 v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function Date: Wed, 22 Oct 2025 12:37:08 -0600 Message-ID: <20251022183717.70829-7-npache@redhat.com> In-Reply-To: <20251022183717.70829-1-npache@redhat.com> References: <20251022183717.70829-1-npache@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Stat-Signature: ntb5m1zgkmutjewcejqu9qjrw3obwhg9 X-Rspamd-Queue-Id: 23C644000C X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1761158367-613578 X-HE-Meta: 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 y9rdGUux G0WeImRDbZ78UTnOA7kRoP6i4+loJL20zWz/e4Pm+WxBLxxLjuN96GnY+1bz3H4qQkv5IzsiYoo5lXq0Wz7ID+TrURVTMDCQ0W6jxCgsugFdTeQBPi8EZUGbeby/PiIhFmhECGu81DLww/1i5qeMaaLzVnVCnilI+YaQapvhGzIilQHwmPNLpy9HYqdg3fwiMjKmVRkOLBpAgY2j7KQDerRZ0rfsB+/LvA+1fhv5CoszVsPRiyQnH+pz3ePpAfjSXp94+BpW9MxDqNd0xHgnVbsxTm07C61xJQNh9ev5U0ngl4pf39GQ3SvTi795yoVrAWio7yJFxOVILa/pwfDrWHKCLi2yg0f4Fja0fcnYqAk+H9XOaxt+TcF1G8wWIqKEwDq9i0ayX8wZsoWKwG7QBrbLtnLDnt09jYzAZ 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 caps the max_ptes_none to HPAGE_PMD_NR / 2 - 1 (255 on 4k page size). The function also scales the max_ptes_none number by the (PMD_ORDER - target collapse order). 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 | 35 ++++++++++++++++++++++++++++++++++- 1 file changed, 34 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 4ccebf5dda97..286c3a7afdee 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -459,6 +459,39 @@ 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, scale down the max_ptes_none proportionally to the folio + * order, but caps it at HPAGE_PMD_NR/2-1 to prevent a collapse feedback loop. + * + * Return: Maximum number of empty PTEs allowed for the collapse operation + */ +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan) +{ + unsigned int max_ptes_none; + + /* ignore max_ptes_none limits */ + if (full_scan) + return HPAGE_PMD_NR - 1; + + if (order == HPAGE_PMD_ORDER) + return khugepaged_max_ptes_none; + + max_ptes_none = min(khugepaged_max_ptes_none, HPAGE_PMD_NR/2 - 1); + + return max_ptes_none >> (HPAGE_PMD_ORDER - order); + +} + void khugepaged_enter_vma(struct vm_area_struct *vma, vm_flags_t vm_flags) { @@ -546,7 +579,7 @@ 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); for (_pte = pte; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) { -- 2.51.0