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Thu, 03 Sep 2026 07:29:39 -0700 (PDT) From: Bo Zhang X-Google-Original-From: Bo Zhang To: akpm@linux-foundation.org, vbabka@kernel.org, david@kernel.org Cc: surenb@google.com, mhocko@suse.com, brendan.jackman@linux.dev, hannes@cmpxchg.org, ziy@nvidia.com, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, zhaonanzhe@xiaomi.com, lipengfei28@xiaomi.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/4] mm: compaction: skip isolating large folios that satisfy the mTHP order Date: Thu, 3 Sep 2026 22:29:23 +0800 Message-Id: <20260903142923.4108729-1-zhangbo56@xiaomi.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260825043833.2659350-3-zhangbo56@xiaomi.com> References: <20260825043833.2659350-3-zhangbo56@xiaomi.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1034A14000E X-Stat-Signature: hjt53g3b1wo11tsuceyj8ofaa1qfxfpd X-Rspam-User: X-HE-Tag: 1788445780-37587 X-HE-Meta: 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 fheaybYP xqcjNnn5K2O6fSDC16tTSeVfQjnsFbB1ptgam33uAdy3zV3WngwK1eiyZYlGskDs+ODM9OP5amyXqPtEKw1kgbBzMwg1FdeqkkuEjmXTisn9bHezjK2dQ3GMNAcIPVr7c6sKxvYVPOj/x0jr+b1YrDCcZT6W/vrZJGn64vPu3/qYHsSx+9Q3Iz0yNj1cuwt5UwaIjpM4Y0Qkb0jvDRISRy+iVD7KC+VV6wLRWfAmLto4QHdCoZ7s/pNM7beMUmv3J/hW553MSU0pyln3Gy4YcEV5oxg4x1sAuBXueKV0RWNgsIYfsBLU6ZuKZFV8HUYAxnNRQrgPTQ7yPSwpEBFd2yrSUim1G9dC176t6 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Thanks for the review. Replying to the two points below. 1) Build breakage when CONFIG_TRANSPARENT_HUGEPAGE is disabled Sashiko says "huge_anon_orders_always is only declared in include/linux/huge_mm.h under CONFIG_TRANSPARENT_HUGEPAGE. Without an #else fallback stub, any kernel build configured with CONFIG_COMPACTION=y and CONFIG_TRANSPARENT_HUGEPAGE=n might fail with an undeclared identifier error." Correct. In v2 the huge_anon_orders_always access is confined to compact_hpage_order() and guarded by #ifdef CONFIG_TRANSPARENT_HUGEPAGE, so this call site no longer references it directly. 2) Skipping isolation harms manual compaction and HugeTLB pool resizing Sashiko says "If smaller mTHPs (like order-2) are enabled, compact_hpage_order() will return 2. This logic will then unconditionally return true for any folio of order 2 or higher, skipping its isolation. Could this cause administrative commands or automated HugeTLB pool resizing scripts to inexplicably fail to allocate large contiguous memory blocks ...?" To clarify the intent first: when mTHP is enabled the proactive compaction target is compact_hpage_order() (e.g. order-2), so folios already at or above that order need not be isolated - migrating smaller fragments is enough to assemble the target block, and isolating larger folios is just wasted work. That said, the problem you describe is real: compact_hpage_order() is derived from huge_anon_orders_always, so it also affects manual `echo 1 > compact_memory` and HugeTLB pool resizing, which legitimately want to form order-9/order-11 blocks. This comes back to a question I raised in the cover letter: what is the right way to determine the compaction target order? Deriving it from huge_anon_orders_always is convenient but conflates mTHP proactive compaction with these other callers. An alternative would be a dedicated knob (e.g. a sysctl compaction target order). I haven't settled on the right approach and would welcome opinions. Bo