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[2003:d8:2f20:da00:b70a:d502:3b51:1f2d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45e01578272sm74377785e9.9.2025.09.12.08.45.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Sep 2025 08:45:29 -0700 (PDT) Message-ID: <59641180-a0d9-400c-aaeb-0c9e93954bf5@redhat.com> Date: Fri, 12 Sep 2025 17:45:26 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support To: Kiryl Shutsemau Cc: Nico Pache , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.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, 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: <20250912032810.197475-1-npache@redhat.com> <43f42d9d-f814-4b54-91a6-3073f7c7cedf@redhat.com> <80c50bf4-27b1-483c-9977-2519369c2630@redhat.com> <7ri4u7uxsv6elyohqiq2w5oxv4yhk2tyniwglfxtiueiyofb3n@l4exlmlf5ty4> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12.09.25 17:41, Kiryl Shutsemau wrote: > On Fri, Sep 12, 2025 at 04:56:47PM +0200, David Hildenbrand wrote: >> On 12.09.25 16:35, Kiryl Shutsemau wrote: >>> On Fri, Sep 12, 2025 at 04:28:09PM +0200, David Hildenbrand wrote: >>>> On 12.09.25 15:47, David Hildenbrand wrote: >>>>> On 12.09.25 14:19, Kiryl Shutsemau wrote: >>>>>> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: >>>>>>> The following series provides khugepaged with the capability to collapse >>>>>>> anonymous memory regions to mTHPs. >>>>>>> >>>>>>> To achieve this we generalize the khugepaged functions to no longer depend >>>>>>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual >>>>>>> pages that are occupied (!none/zero). After the PMD scan is done, we do >>>>>>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD >>>>>>> range. The restriction on max_ptes_none is removed during the scan, to make >>>>>>> sure we account for the whole PMD range. When no mTHP size is enabled, the >>>>>>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled >>>>>>> by the attempted collapse order to determine how full a mTHP must be to be >>>>>>> eligible for the collapse to occur. If a mTHP collapse is attempted, but >>>>>>> contains swapped out, or shared pages, we don't perform the collapse. It is >>>>>>> now also possible to collapse to mTHPs without requiring the PMD THP size >>>>>>> to be enabled. >>>>>>> >>>>>>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on >>>>>>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for >>>>>>> mTHP collapses to prevent collapse "creep" behavior. This prevents >>>>>>> constantly promoting mTHPs to the next available size, which would occur >>>>>>> because a collapse introduces more non-zero pages that would satisfy the >>>>>>> promotion condition on subsequent scans. >>>>>> >>>>>> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count >>>>>> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. >>>>> >>>>> BTW, I thought further about this and I agree: if we count zero-filled >>>>> pages towards none_or_zero one we can avoid the "creep" problem. >>>>> >>>>> The scanning-for-zero part is rather nasty, though. >>>> >>>> Aaand, thinking again from the other direction, this would mean that just >>>> because pages became zero after some time that we would no longer collapse >>>> because none_or_zero would then be higher. Hm .... >>>> >>>> How I hate all of this so very very much :) >>> >>> This is not new. Shrinker has the same problem: it cannot distinguish >>> between hot 4k that happened to be zero from the 4k that is there just >>> because of we faulted in 2M a time. >> >> Right. And so far that problem is isolated to the shrinker. >> >> To me so far "none_or_zero" really meant "will I consume more memory when >> collapsing". That's not true for zero-filled pages, obviously. > > Well, KSM can reclaim these zero-filled memory until we collapse it. KSM is used so rarely (for good reasons) that I would never ever build an argument based on its existence :P But yes: during the very first shrinker discussion I raised that KSM can do the same thing. Obviously that was not good enough. -- Cheers David / dhildenb