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t=1780489640; bh=uNfk9BcPdly9aVTPrznds3uJR0OWgZjLs5X0wKssQG0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=cl2NQkYCVfoihPRZMFzihOK/KzDr1MQWeGdKvH2VD3xbr9ngUyMrQgHpzHxo87puf Uhc5ydifJ8z/cw9MetJNzvcVZNyi66FkiF0YbAKt7EIBqwk7ea4vKb0HlftPvy10HN C5YrywpP4xwhXtmU3t7M8AM+iDyIy8WGdebGnJvPIRpNQvy8QAV/pwCkWvFKcuu1kj fsMx7i3HnwJJVyZB5QDnts8G8bCvhveQFYQxh4Lsh5p3kE5nt2NAmLyZyDxQm1G7nY WTzWZVluCtSXURw5yiWS+s+iLozVM0Oa+XNTHfXV4/4p5Z5qEkdbVXB/x0BeKzXmel DL7doHtiYgeTA== Message-ID: <053db37c-bafa-43b2-ac54-717e461b57a8@kernel.org> Date: Wed, 3 Jun 2026 14:27:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 11/14] mm/khugepaged: Introduce mTHP collapse support To: Nico Pache Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, 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, 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, Usama Arif , usamaarif642@gmail.com References: <20260522150009.121603-1-npache@redhat.com> <20260522150009.121603-12-npache@redhat.com> <19639b08-5bf1-4974-9635-c458d512fa38@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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>>> - const u16 mid_offset = offset + (nr_ptes / 2); >>> - >>> - collapse_mthp_stack_push(cc, &stack_size, mid_offset, >>> - next_order); >>> - collapse_mthp_stack_push(cc, &stack_size, offset, >>> - next_order); >>> + if (order > KHUGEPAGED_MIN_MTHP_ORDER && >>> + (BIT(order) - 1) & enabled_orders) { >> >> Why not a test_bit() ? > > The test bit is at the top of the loop. This adds a exit if the lower > orders are all disabled or we hit the last order. Ah, now I understand what you want to do here. I guess you should add a () around the & and maybe add a comment. And likely using GENMASK is clearer? /* * Continue with the next smaller order if there is still * any smaller order enabled. */ if (order > KHUGEPAGED_MIN_MTHP_ORDER && (enabled_orders & GENMASK(order - 1, 0))) { ... } > >> >> >> But, wouldn't you want to skip orders that are not enabled and try with the next >> smaller one in any case before you advance the offset? > > We are currently iterating through each order (not skipping them). > There may be optimizations to avoid iterating through every order > (like your changes suggest), but currently, every collapse, whether it > succeeds or fails at the bottom order, must also iterate the offset. Right, we currently miss opportunities to just skip, that we can optimize later. -- Cheers, David