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[80.81.2.50]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4771e18bd92sm5763515e9.2.2025.10.28.11.17.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Oct 2025 11:17:18 -0700 (PDT) Message-ID: Date: Tue, 28 Oct 2025 19:17:16 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function To: Lorenzo Stoakes Cc: Baolin Wang , Nico Pache , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, ziy@nvidia.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 References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-7-npache@redhat.com> <5f8c69c1-d07b-4957-b671-b37fccf729f1@lucifer.local> <063f8369-96c7-4345-ab28-7265ed7214cb@linux.alibaba.com> <9a3f2d8d-abd1-488c-8550-21cd12efff3e@lucifer.local> <64b9a6cd-d2e4-4142-bf41-abe80bf1f61a@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 28.10.25 19:09, Lorenzo Stoakes wrote: > (It'd be good if we could keep all the 'solutions' in one thread as I made a > detailed reply there and now all that will get lost across two threads but > *sigh* never mind. Insert rant about email development here.) Yeah, I focused in my other mails on things to avoid creep while allowing for mTHP collapse. > > On Tue, Oct 28, 2025 at 06:56:10PM +0100, David Hildenbrand wrote: >> [...] >> >>> >>>> towards David's earlier simplified approach: >>>> max_ptes_none == 511 -> collapse mTHP always >>>> max_ptes_none != 511 -> collapse mTHP only if all PTEs are non-none/zero >>> >>> Pretty sure David's suggestion was that max_ptes_none would literally get set to >>> 511 if you specified 511, or 0 if you specified anything else. >> >> We had multiple incarnations of this approach, but the first one really was: >> >> max_ptes_none == 511 -> collapse mTHP always > > But won't 511 mean we just 'creep' to maximum collapse again? Does that solve > anything? No creep, because you'll always collapse. Creep only happens if you wouldn't collapse a PMD without prior mTHP collapse, but suddenly would in the same scenario simply because you had prior mTHP collapse. At least that's my understanding. > >> max_ptes_none == 0 -> collapse mTHP only if all non-none/zero >> >> And for the intermediate values >> >> (1) pr_warn() when mTHPs are enabled, stating that mTHP collapse is not >> supported yet with other values > > It feels a bit much to issue a kernel warning every time somebody twiddles that > value, and it's kind of against user expectation a bit. pr_warn_once() is what I meant. > > But maybe it's the least worst way of communicating things. It's still > absolutely gross. > >> (2) treat it like max_ptes_none == 0 or (maybe better?) just disable mTHP >> collapse > > Yeah disabling mTHP collapse for these values seems sane, but it also seems that > we should be capping for this to work correctly no? I didn't get the interaction with capping, can you elaborate? > > Also I think all this probably violates requirements of users who want to have > different behaviour for mTHP and PMD THP. > > The default is 511 so we're in creep territory even with the damn default :) I don't think so, but maybe I am wrong. -- Cheers David / dhildenb