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[2003:d8:2f09:9c00:8173:2a94:640d:dd31]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b6f306c93sm313894595e9.14.2025.09.03.01.13.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Sep 2025 01:13:36 -0700 (PDT) Message-ID: <759bff7a-3918-41ac-a184-8c07ec414bb2@redhat.com> Date: Wed, 3 Sep 2025 10:13:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs To: Wei Yang , Dev Jain 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20250903080839.wuivg2u7smyuxo5e@master> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: VDJjw5Iulhep--qAaIFtXyTxStYwrYENqEu00-Z1iNU_1756887218 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: AC12F1C000B X-Stat-Signature: qkmsndcusung83bdii6ffnw1gbmud89h X-Rspam-User: X-HE-Tag: 1756887222-924550 X-HE-Meta: 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 Cnlu9uFp 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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 10:08, 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? The max_ptes_shared check should handle that, so I don't immediately see a problem with that. When I thought about the "why is this writable check there" in the past, I thought that maybe it was "smarter" to use THP where people are actually using that memory for writing (writing heap etc). But I can understand that some pure r/o users exists that can benefit as well. -- Cheers David / dhildenb