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Thu, 03 Sep 2026 07:11:29 -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 1/4] mm: compaction: make proactive compaction mTHP-aware Date: Thu, 3 Sep 2026 22:11:15 +0800 Message-Id: <20260903141115.4107677-1-zhangbo56@xiaomi.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260825043833.2659350-2-zhangbo56@xiaomi.com> References: <20260825043833.2659350-2-zhangbo56@xiaomi.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 4F263C0008 X-Stat-Signature: 4xz5413smjza9mrxdgxttbgah9hh1egj X-HE-Tag: 1788444691-608717 X-HE-Meta: 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 FFVvBu3W Ps+rl5di0P5Qi/ssvJ0r6+ygZpIyWGc4bOz3nSaHKUj+Yjl8XrvT6EsTJBYnpD0PwwF1C3gWrrPmRu0gsbrV4BEqG3rtssZD3Zm6Z/Lf9Bn9SSJniXwoU+bs0beIjHPEIqZXIz9Yk8AcEm3LjCdSbQfc6JgLI8mmvAWOMOuXCWgwxhPSw00YVveQXyQLMJ+x0Mt/eleLd5ZYRcBJNtIMDRLoZSPqSVa6OI/4d6bMqWZdWn9qbm/sgit7mwWQc7Kyur3VPly1wviaQvphSUN0fApNoPn71qCzJW56yj8PWBBPwiuQ42FD1C46NNzJjUpoU5MHJkSm+gJlGIQaz/WHFsmoU6jEHvd3mhW5o 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 three points below. 1) Build breakage when CONFIG_TRANSPARENT_HUGEPAGE is disabled Sashiko says "Does this break the build when CONFIG_TRANSPARENT_HUGEPAGE is disabled? In include/linux/huge_mm.h, the declaration of huge_anon_orders_always is guarded by #ifdef CONFIG_TRANSPARENT_HUGEPAGE, but here it is referenced unconditionally." You're right (the kernel test robot reported the same on m68k). In v2 I'll guard the access with #ifdef CONFIG_TRANSPARENT_HUGEPAGE and fall back to COMPACTION_HPAGE_ORDER. 2) __ffs() singles out the lowest enabled mTHP order Sashiko says "If an administrator enables both order-2 and order-9 THPs, it seems this will return order-2, and __compact_finished() will halt compaction early once enough order-2 blocks are formed, potentially starving the system of order-9 THPs." Yes, this is a deliberate simplification, also raised as an open question in the cover letter. I don't have a firm answer on the best policy for multiple enabled orders yet and would welcome opinions. 3) Racing with concurrent sysfs updates Sashiko says "kcompactd() caches the target order ... However, __compact_finished() dynamically re-reads it here. If the target order changes mid-compaction ... Could this mismatch falsely indicate a lack of progress and erroneously defer proactive compaction?" Good catch. The clean fix would be to read the order once and plumb it through the compaction path. But during proactive compaction target_order is -1, and I couldn't find a clean way to pass the mTHP order down - which is why the code reads huge_anon_orders_always directly. This is the open question I raised in the cover letter; suggestions welcome. Bo