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[2003:d8:2f09:9c00:8173:2a94:640d:dd31]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b7e898b99sm231483255e9.19.2025.09.03.02.22.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Sep 2025 02:22:11 -0700 (PDT) Message-ID: <99f8cb5e-828a-444c-b207-2a12e13a45f5@redhat.com> Date: Wed, 3 Sep 2025 11:22:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs To: Dev Jain , Wei Yang Cc: akpm@linux-foundation.org, kas@kernel.org, willy@infradead.org, hughd@google.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250903054635.19949-1-dev.jain@arm.com> <20250903080839.wuivg2u7smyuxo5e@master> <0a52cb54-5633-4374-baa5-199194dfc2e1@arm.com> <3bd9ff5d-de8a-469f-a7b0-41c192b23993@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 4929018000E X-Stat-Signature: m8k5id7nbx8h9u5z6ayhg4oaj9mp1r4c X-HE-Tag: 1756891337-324663 X-HE-Meta: U2FsdGVkX1/dALdVt4w0dISlFw+/pgVblsSuQkhZGINyn9rNXyeBKM0rV4eay6OixVXzzTdmU2Pdrr+3TbqdCFXZG51kCApZCRZUhzEJDdzZ23Lb90AXiZsP+JS6Te3u+vLly/TnvgqK+f4ilSsDHMcseO2jlVD+s5lHtfXV93h7/y8DdfG94+p4sJFI+bHGCETXQdVgGRVq+wpBd5pKWByjfXeZpB69Sb+znnQ0qiC4+/px8JVOtKyOJL4cMc46iuDcSHF1mThodzKZ+VmvB1s7uUlhq3e3ZWoH3WGcn+b2VNpPIehWY5CGpqZc1VLy7Wg9bkXEWHvq6PtBMQzavZQozjjj8DaR15BudxWcg9YEQZKXtBsnfCtTy9Da4AZOss1HXVByo9nzSBZ0GyV5SySHea7+PgexkTHriAPnkSRFO/ATNxNGOxawF+GLzlyXv1eDqw8BRYfBBCiXejmDLo9CeUl38VCVqq19XudRicz8qroK2IT3/BkQnTB834paF8ZvV1pu2Jr0+byJPYy6HfEcfniFBM50WeaeQ15tgNr+Sr1ZGOtZp9Sf1+7kBu8drB3zTPy7n6TIoiTcTRhz7IlS0p4ja2Cal+DyEfanXoLcNuBTfIBscKEY3bjI7a6TjfFOCRDmfy8mM48TJnM8EGgqcsenzCxCvo0VuBOuHFDnYGuyAxmFu7IeGkjDIfUhUnmnZWDt2qJZAxr9XQbebaH9pI9/svQObvV3opKv9w/R2J1SJXQG+sTNCSBYLzb4TVMdj9MmOt7Uc1HJv2qELa7evP15Y5rz3thXH+I5NtIys2lwZyRZ7MQ59QW9/3ZoRdRyNRQl5uqUF7hn8fewk1nYdBvuA76Yk7Ci0bhRjoMSCBq0tm9OxI3ua9XoQJqhC/OmUyPTMqUrxCy0KBHYCW0lr8nJtRKL0huJId8z7KUrEjLWNNiqGgGTWonjd/DzAqEM1R/kHmr5jzMUefi dCEVLcqb 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03.09.25 11:18, Dev Jain wrote: > > On 03/09/25 2:45 pm, Dev Jain wrote: >> >> On 03/09/25 1:38 pm, Wei Yang wrote: >>> On Wed, Sep 03, 2025 at 11:16:34AM +0530, Dev Jain wrote: >>>> Currently khugepaged does not collapse a region which does not have a >>>> single writable page. This is wasteful since non-writable VMAs >>>> mapped by >>>> the application won't benefit from THP collapse. Therefore, remove this >>>> restriction and allow khugepaged to collapse a VMA with arbitrary >>>> protections. >>>> >>>> Along with this, currently MADV_COLLAPSE does not perform a collapse >>>> on a >>>> non-writable VMA, and this restriction is nowhere to be found on the >>>> manpage - the restriction itself sounds wrong to me since the user >>>> knows >>>> the protection of the memory it has mapped, so collapsing read-only >>>> memory via madvise() should be a choice of the user which shouldn't >>>> be overriden by the kernel. >>>> >>>> On an arm64 machine, an average of 5% improvement is seen on some >>>> mmtests >>>> benchmarks, particularly hackbench, with a maximum improvement of 12%. >>>> >>>> Signed-off-by: Dev Jain >>>> --- >>> [...] >>>> mm/khugepaged.c | 9 ++------- >>>> 1 file changed, 2 insertions(+), 7 deletions(-) >>>> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index 4ec324a4c1fe..a0f1df2a7ae6 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct >>>> vm_area_struct *vma, >>>>             writable = true; >>>>     } >>>> >>>> -    if (unlikely(!writable)) { >>>> -        result = SCAN_PAGE_RO; >>>> -    } else if (unlikely(cc->is_khugepaged && !referenced)) { >>> Would this cause more memory usage in system? >>> >>> For example, one application would fork itself many times. It >>> executable area >>> is read only, so all of them share one copy in memory. >>> >>> Now we may collapse the range and create one copy for each process. >>> >>> Ok, we have max_ptes_shared, while if some ptes are none, could it >>> still do >>> collapse? >>> >>> Maybe this is not realistic, just curious. >> >> Misunderstood your concern - you mean to say that a parent forks and >> the children >> VMAs are read-only pointing to the pages which were mapped by parent. >> Hmm. > > I meant to say, writable VMAs with wrprotected ptes. Maybe after this > patch, people > > can finally make some real use of the max_ptes_shared tunable :) I hope not, because it should be burned with fire, lol :) -- Cheers David / dhildenb