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b=iva+R/ZuCZyU82gLI0nCN8tz9gUnpWYVgMWLdSU4nhRi3x92j+Y+gTyAKpLdj3WOb gkcSQKfgLkV6Rg7AL4zeWhvck5bGRMuADD12WGF1m5lJnr4mnSd6QeMKg9wVCbSqnu sHYC4p3D+Wigx8tG8PWSZ7txGTjeJQP7TQ0kNe/p1NXPKPfpfVECpASqlzxqnlbyFI i1arl/yhuLMLVC7iKUZKs3t+HiLY6nI/M7SacKTo3Lm3xHtioBKUG0GhuaPF5RpvIp yqUIdJWmtkKsbAl5dpESapafjJwL7Pme6s5YQqUv9uVxltTMgNa7ewiXRc0l7gtMwr z5euTjUeXnlbw== Message-ID: Date: Mon, 21 Sep 2026 12:22:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/3] mm: split underused anonymous mTHP folios To: Usama Arif , Lance Yang Cc: joannelkoong@gmail.com, akpm@linux-foundation.org, ljs@kernel.org, hannes@cmpxchg.org, baohua@kernel.org, alex@ghiti.fr, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, willy@infradead.org, linux-mm@kvack.org, kas@kernel.org References: <20260921100852.2236761-1-usama.arif@linux.dev> 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-Rspam-User: X-Rspamd-Queue-Id: 36EF6180003 X-Stat-Signature: sdddfgbis3chsg6aewn347iggzr3qrzt X-Rspamd-Server: rspam01 X-HE-Tag: 1789986130-590842 X-HE-Meta: U2FsdGVkX1/fG0wSqcO4vX/Qzt5SGe2Cbd4PiY4OvQgyT2eiMvICWcfxPsec4TdVoKatRoGujSYWg6ZakKHhPvSyv0lq4PQuBtOP7Sce1+i6TW7/kbn4fRUsLknKtkUN3GWGr/O3rbA1MpKeobtM0ruJKg3BHns7mj+K+C35HuQK9oHhsqtgiulo0/Lbn0DLXu42neENQsqFcVJf03Nts1FXhEDzYI+/NkozFiQkAnyE0Laa0RVcr4NaNfSWYm29OhgwV0hwizIZqVssQZfYFoS2pIH6uWZNxBV5ctaRlZ3yLcIsqQpis9n/Zys/cfF0nnBq+IVmUpBOSHMVfQg8YrJ8ogSwvxYGtYog+phkBRW7b4YwLXIwwHFInvc9XMQ+hO1i907Fg1YDQOFkRfqz7JXwM8Hzy+Moqr/LJsJ+7FAYZM0Dbgil3ulzijhLu4LjWBhG4SbbyEWeVmHLxRMkN58DztmdjTgDBtmv8oWwfyIpR+Cp0cTmdiCbqaOmIvv+CU5ykaeBZVrXvDcsFiHGHs8nE+MPqWg8/5zo2rHguo8X0VucpyKd/Ir8DOfxiS+yYGs7+qkW6pyzJ30OM2HaVXNMF80MwgezG75NJ7spvaJJCUwfxAGo0BsfN7Ypvz62Tc3OEw+4gFKc4dQapnba0WMWQf5nhEp0m7esj3Ql1X95ao4M4qb/HJ0tFdNjqXAsvXCWt+M8YquUy2ZM5cN2yswhUiRD/t/SSQtj9Wj3YjWbW+2l35i96Cjg0iSlvWx8/7hlLFQ5qXvFGWDGLjoQ5aMAPWrTHLJd7KslT5zvrCq3WJ8elnmKv9GdxAwmyEseLRiGc5xOapz1CYFOjwHKN4UdmrAwfa62MAcSQFHER00MmAr7LaFpyjlF9XeFs4K9prTD+KRlBjzz/erButUjHkLD9ysn5pweN0qmwqvCMBJV4tJfxra6c16WnVcxJUfLFhPNGyK27iPXViAwoa9 l/7eViV0 z+p9G5rZAkyqq66j8aTq3IXw1G8bvz3ACLgyxjCnr2xgqB7dGxh5jzJzI9/4YPHIGis8FuueA3IizmEAg7jEjiDIUvq3DHEtX9krE5i+Lod8qzqG+nduLKgbxYh7qx99I6KbiZr2D1FDXPrJKD2w4XPx59/uv1+GEYKsI8t/+qotCw9rGZr3tM8Uw8oej71RsSH4VJ3EOQDOlk1UZ3S1OnOBfaRt2JcVivWVFx8eUEIWYukBgMYC0s2FP+xpDQ/RUAtKECwvIS0GBTEE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > Kiryl, would your rework allow all values for mTHP collapse or would > it stil only allow HPAGE_PMD_NR = 1 and 0? > >> Would a per-order value make more sense here? >> > > I feel like per-order might be too many knobs? What about a percentage > that might apply to all mTHP orders? We discussed all that in the past, and concluded that we will have to support the existing toggle. I played with using a percentage, but having two toggles possibly mean the same thing (and some values not being possible) turned ugly. So the conclusion for me was: no new toggles. > >> An unset value could preserve the current behavior, while an explicit >> value could control both mTHP collapse and underused splitting. That > > Ah are you proposing that mTHP collapse can then take all values from > 0 to mTHP number of pages? > I think one of the inital version of Nicos patches for mTHP collpase > allowed that (hopefully I am not misremembering). Nico, what was the > reason for only allowing 0 and max? The creep problem was discussed plenty of times. That's when we decided to start with 0 and max, and only add support for scaling later, once we actually know which use cases would need them. Without support for deferred shrinker, there wasn't really the need to support other values. Some of it can be had in: commit 5a9c05d86683db5449b7fefd278c461cffb55234 Author: Nico Pache Date: Fri Jun 5 10:14:11 2026 -0600 mm/khugepaged: generalize __collapse_huge_page_* for mTHP support generalize the order of the __collapse_huge_page_* and collapse_max_* functions to support future mTHP collapse. The current mechanism for determining collapse with the khugepaged_max_ptes_none value is not designed with mTHP in mind. This raises a key design issue: if we support user defined max_pte_none values (even those scaled by order), a collapse of a lower order can introduces an feedback loop, or "creep", when max_ptes_none is set to a value greater than HPAGE_PMD_NR / 2. [1] With this configuration, a successful collapse to order N will populate enough pages to satisfy the collapse condition on order N+1 on the next scan. This leads to unnecessary work and memory churn. To fix this issue introduce a helper function that will limit mTHP collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. This effectively supports two modes: [2] - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE that maps the shared zeropage. Consequently, no memory bloat. - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest available mTHP order. This removes the possibility of "creep", and a warning will be emitted if any non-supported max_ptes_none value is configured with mTHP enabled. Any intermediate value will default mTHP collapse to max_ptes_none=0. mTHP collapse will not honor the khugepaged_max_ptes_shared or khugepaged_max_ptes_swap parameters, and will fail if it encounters a shared or swapped entry. Tracing back the discussion on that patch should give you all the details. -- Cheers, David