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imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Z+e1iPjf; spf=pass (imf28.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1780682602; b=W64tmiF7JN6O+x2ths0lohZteoR9hu15IxkzPREdrHUdr3G1Tw6Bq9xvcwM8S8DdXH40ij 7qFPLGGotMqWp05B2NxHWYuVqp43VTK12PYM878Hs3psJZDncpJy/8aqFYbb7kCdyqgwCh Oyb2DZ0Oh/rlzTaSluiXK4T/HlOvpdc= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id A0AB340A3D; Fri, 5 Jun 2026 18:03:20 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BEDA81F00893; Fri, 5 Jun 2026 18:03:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780682600; bh=PGzu0aXe6HFW26JonkhpJzsdo5T0zMBSmF+3OrFmmlw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Z+e1iPjfg+9HqlR3h8DHfBgPQRE68RxUVv8D1XNuenBu9ZF2YKFA1VjR+VhNGmsjH GFEsnohYRLoqPHInWHvAv+Y1P65hv1c/MMrxJSbBN+yOfJqPCciWJB5eDfz41GF86T qMqtn3TPL6lAtshubXlzylKeK/mZnTMTuJ8OreQfyscHUNmR0JCN2NxoetsjDP9SNX z6t2qhgbRR6iVRlYl5H6U92DpJ60wb1xEgXi0Mh5HIK/ZHtPmOapC6wlHz+l+codie TgFnFCzdg4BQYDwbENybXouCX/1iPohb/lA5DoazHpV+yJs7xg5DN0jxXw7ub4HMGJ C0hR8pP7Fim6A== Message-ID: Date: Fri, 5 Jun 2026 20:03:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support To: Nico Pache , 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, 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@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.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 References: <20260605161422.213817-1-npache@redhat.com> <20260605161422.213817-12-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: C6FACC0010 X-Stat-Signature: yr5gjykgctgai7zebpetbejfz76nhoxy X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1780682601-845593 X-HE-Meta: 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 oCJSentP 4P7iHzGGUnmHD4pxjG9tBoP9QAqN8PgappM9/ttAQ4fw1LvDYdpXctc67baOjAkcDdPTlIkSGfs0cums5Vjah64wSIx0qrNhXqY+ja9PJ6bo8aUSHnmSZS+1Rl2bnvoZ6RdumuHIdSbfTkBQHxuShClT1w1/+QgRjdAyCT5X+fDV8dNoCbyoZP4pjX7B/rnoZ4mLcWu4DRjSWpOU8rA5vRUO4HMUi3OkX6nbUTjepA0GmpEDKWml4ve+R1gLf+E1UOTfR1OwEWi0Mp5pfQRabktNGn5pgihWW1LK1axBkBRnjx8NWNWDk8xPnXPzX586r5F1kZ7N/9OkTMjLMIyHXDaeixSXpcf7EBax94Mi/f2fYU6Ack7T94WCP1VDkfrLXS0N4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/5/26 18:14, Nico Pache wrote: > Enable khugepaged to collapse to mTHP orders. This patch implements the > main scanning logic using a bitmap to track occupied pages and the > algorithm to find optimal collapse sizes. > > Previous to this patch, PMD collapse had 3 main phases, a light weight > scanning phase (mmap_read_lock) that determines a potential PMD > collapse, an alloc phase (mmap unlocked), then finally heavier collapse > phase (mmap_write_lock). > > To enabled mTHP collapse we make the following changes: > > During PMD scan phase, track occupied pages in a bitmap. When mTHP > orders are enabled, we remove the restriction of max_ptes_none during the > scan phase to avoid missing potential mTHP collapse candidates. Once we > have scanned the full PMD range and updated the bitmap to track occupied > pages, we use the bitmap to find the optimal mTHP size. > > Implement mthp_collapse() to walk forward through the bitmap and > determine the best eligible order for each naturally-aligned region. The > algorithm starts at the beginning of the PMD range and, for each offset, > tries the highest order that fits the alignment. If the number of > occupied PTEs in that region satisfies the max_ptes_none threshold for > that order, a collapse is attempted. On failure, the order is > decremented and the same offset is retried at the next smaller size. Once > the smallest enabled order is exhausted (or a collapse succeeds), the > offset advances past the region just processed, and the next attempt > starts at the highest order permitted by the new offset's natural > alignment. > > The algorithm works as follows: > 1) set offset=0 and order=HPAGE_PMD_ORDER > 2) if the order is not enabled, go to step (5) > 3) count occupied PTEs in the (offset, order) range using > bitmap_weight_from() > 4) if the count satisfies the max_ptes_none threshold, attempt > collapse; on success, advance to step (6) > 5) if a smaller enabled order exists, decrement order and retry > from step (2) at the same offset > 6) advance offset past the current region and compute the next > order from the new offset's natural alignment via __ffs(offset), > capped at HPAGE_PMD_ORDER > 7) repeat from step (2) until the full PMD range is covered > > mTHP collapses reject regions containing swapped out or shared pages. > This is because adding new entries can lead to new none pages, and these > may lead to constant promotion into a higher order mTHP. A similar > issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse > introducing at least 2x the number of pages, and on a future scan will > satisfy the promotion condition once again. This issue is prevented via > the collapse_max_ptes_none() function which imposes the max_ptes_none > restrictions above. > > We currently only support mTHP collapse for max_ptes_none values of 0 > and HPAGE_PMD_NR - 1. resulting in the following behavior: > > - max_ptes_none=0: Never introduce new empty pages during collapse > - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest > available mTHP order > > Any other max_ptes_none value will emit a warning and default mTHP > collapse to max_ptes_none=0. There should be no behavior change for PMD > collapse. > > Once we determine what mTHP sizes fits best in that PMD range a collapse > is attempted. A minimum collapse order of 2 is used as this is the lowest > order supported by anon memory as defined by THP_ORDERS_ALL_ANON. > > Currently madv_collapse is not supported and will only attempt PMD > collapse. > > We can also remove the check for is_khugepaged inside the PMD scan as > the collapse_max_ptes_none() function handles this logic now. > > Signed-off-by: Nico Pache > --- Yeah, overall much simpler and much easier to get. As discussed, we can optimize this later to traverse enabled orders more efficiently. > + bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE); > memset(cc->node_load, 0, sizeof(cc->node_load)); > nodes_clear(cc->alloc_nmask); > + > + enabled_orders = collapse_possible_orders(vma, vma->vm_flags, tva_flags); > + > + /* > + * If PMD is the only enabled order, enforce max_ptes_none, otherwise > + * scan all pages to populate the bitmap for mTHP collapse. > + */ I think it would have been good to mention where the check is performed for mTHP collapse. Can be added later. Acked-by: David Hildenbrand (Arm) -- Cheers, David