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[2003:d8:2f37:2b00:948c:dd9f:29c8:73f4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b6f30fe02sm222458785e9.18.2025.09.01.01.32.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 01 Sep 2025 01:32:11 -0700 (PDT) Message-ID: <620f01b4-8a6c-4dac-bcdd-7c48f5cd87f2@redhat.com> Date: Mon, 1 Sep 2025 10:32:10 +0200 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: [RFC PATCH] mm: Enable khugepaged to operate on non-writable VMAs To: Dev Jain , akpm@linux-foundation.org, kas@kernel.org, willy@infradead.org, hughd@google.com Cc: 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: <20250901074817.73012-1-dev.jain@arm.com> From: David Hildenbrand Content-Language: en-US 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: <20250901074817.73012-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 01.09.25 09:48, Dev Jain wrote: > Currently khugepaged does not collapse a region which does not have a > single writable page. This is wasteful since, apart from any non-writable > memory mapped by the application, there are a lot of non-writable VMAs > which will benefit from collapsing - the VMAs of the executable, those > of the glibc, vvar and vdso, which won't be unmapped during the lifetime > of the process, as opposed to other VMAs which maybe unmapped. Are these anonymous folios? ("VMAs of the executable"), or is you description misleading? > 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. > > I dug into the history of this and couldn't find any concrete reason of > the current behaviour - [1] is the v1 of the original khugepaged patch > which required all ptes to be writable. [2] is the v1 of the patch which > changed this behaviour to require at least one pte to be writable. The > closest thing I could find was: in response to [2], Kirill says in [3] - > "As a side effect it will effectively allow collapse in PROT_READ vmas, > right? I'm not convinced it's a good idea." (Although Kirill realizes in > [4] that this was not the intention of the patch). > > I can see performance improvements on mmtests run on an arm64 machine > comparing with 6.17-rc2. (I) denotes statistically significant improvement, > (R) denotes statistically significant regression (Please ignore the > numbers in the middle column): I once dug into that myself as well as part of commit 1bafe96e89f056cb6e25d47451fb16aee2c7c4d0 Author: David Hildenbrand Date: Wed Apr 24 14:26:30 2024 +0200 mm/khugepaged: replace page_mapcount() check by folio_likely_mapped_shared() where I noted: Interestingly, khugepaged will only collapse an anonymous THP if at least one PTE is writable. After fork(), that means that something (usually a page fault) populated at least a single exclusive anonymous THP in that PMD range. The problem I was concerned with (also documented in that patch) should no longer apply ever since we changed how folio_maybe_mapped_shared() operates. So yes, I don't see a good reason to fail on R/O PTEs > > +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+ > | mmtests/hackbench | process-pipes-1 (seconds) | 0.145 | -0.06% | > | | process-pipes-4 (seconds) | 0.4335 | -0.27% | > | | process-pipes-7 (seconds) | 0.823 | (I) -12.13% | > | | process-pipes-12 (seconds) | 1.3538333333333334 | (I) -5.32% | > | | process-pipes-21 (seconds) | 1.8971666666666664 | (I) -2.87% | > | | process-pipes-30 (seconds) | 2.5023333333333335 | (I) -3.39% | > | | process-pipes-48 (seconds) | 3.4305 | (I) -5.65% | > | | process-pipes-79 (seconds) | 4.245833333333334 | (I) -6.74% | > | | process-pipes-110 (seconds) | 5.114833333333333 | (I) -6.26% | > | | process-pipes-141 (seconds) | 6.1885 | (I) -4.99% | > | | process-pipes-172 (seconds) | 7.231833333333334 | (I) -4.45% | > | | process-pipes-203 (seconds) | 8.393166666666668 | (I) -3.65% | > | | process-pipes-234 (seconds) | 9.487499999999999 | (I) -3.45% | > | | process-pipes-256 (seconds) | 10.316166666666666 | (I) -3.47% | > | | process-sockets-1 (seconds) | 0.289 | 2.13% | > | | process-sockets-4 (seconds) | 0.7596666666666666 | 1.02% | > | | process-sockets-7 (seconds) | 1.1663333333333334 | -0.26% | > | | process-sockets-12 (seconds) | 1.8641666666666665 | -1.24% | > | | process-sockets-21 (seconds) | 3.0773333333333333 | 0.01% | > | | process-sockets-30 (seconds) | 4.2405 | -0.15% | > | | process-sockets-48 (seconds) | 6.459666666666666 | 0.15% | > | | process-sockets-79 (seconds) | 10.156833333333333 | 1.45% | > | | process-sockets-110 (seconds) | 14.317833333333333 | -1.64% | > | | process-sockets-141 (seconds) | 20.8735 | (I) -4.27% | > | | process-sockets-172 (seconds) | 26.205333333333332 | 0.30% | > | | process-sockets-203 (seconds) | 31.298000000000002 | -1.71% | > | | process-sockets-234 (seconds) | 36.104000000000006 | -1.94% | > | | process-sockets-256 (seconds) | 39.44016666666667 | -0.71% | > | | thread-pipes-1 (seconds) | 0.17550000000000002 | 0.66% | > | | thread-pipes-4 (seconds) | 0.44716666666666666 | 1.66% | > | | thread-pipes-7 (seconds) | 0.7345 | -0.17% | > | | thread-pipes-12 (seconds) | 1.405833333333333 | (I) -4.12% | > | | thread-pipes-21 (seconds) | 2.0113333333333334 | (I) -2.13% | > | | thread-pipes-30 (seconds) | 2.6648333333333336 | (I) -3.78% | > | | thread-pipes-48 (seconds) | 3.6341666666666668 | (I) -5.77% | > | | thread-pipes-79 (seconds) | 4.4085 | (I) -5.31% | > | | thread-pipes-110 (seconds) | 5.374666666666666 | (I) -6.12% | > | | thread-pipes-141 (seconds) | 6.385666666666666 | (I) -4.00% | > | | thread-pipes-172 (seconds) | 7.403000000000001 | (I) -3.01% | > | | thread-pipes-203 (seconds) | 8.570333333333332 | (I) -2.62% | > | | thread-pipes-234 (seconds) | 9.719166666666666 | (I) -2.00% | > | | thread-pipes-256 (seconds) | 10.552833333333334 | (I) -2.30% | > | | thread-sockets-1 (seconds) | 0.3065 | (R) 2.39% | > +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+ > > +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+ > | mmtests/sysbench-mutex | sysbenchmutex-1 (usec) | 194.38333333333333 | -0.02% | > | | sysbenchmutex-4 (usec) | 200.875 | -0.02% | > | | sysbenchmutex-7 (usec) | 201.23000000000002 | 0.00% | > | | sysbenchmutex-12 (usec) | 201.77666666666664 | 0.12% | > | | sysbenchmutex-21 (usec) | 203.03 | -0.40% | > | | sysbenchmutex-30 (usec) | 203.285 | 0.08% | > | | sysbenchmutex-48 (usec) | 231.30000000000004 | 2.59% | > | | sysbenchmutex-79 (usec) | 362.075 | -0.80% | > | | sysbenchmutex-110 (usec) | 516.8233333333334 | -3.87% | > | | sysbenchmutex-128 (usec) | 593.3533333333334 | (I) -4.46% | > +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+ > > No regressions were observed with mm-selftests. > > [1] https://lore.kernel.org/all/679861e2e81b32a0ae08.1264054854@v2.random/ > [2] https://lore.kernel.org/all/1421999256-3881-1-git-send-email-ebru.akagunduz@gmail.com/ > [3] https://lore.kernel.org/all/20150123113701.GB5975@node.dhcp.inet.fi/ > [4] https://lore.kernel.org/all/20150123155802.GA7011@node.dhcp.inet.fi/ > > Signed-off-by: Dev Jain > --- > Based on mm-new. > > Not very sure of the tracing parts which this patch changes. I have kept > the writable portion for the tracing to maintain backward compat, just > dropped it as a collapse condition. > > include/trace/events/huge_memory.h | 2 +- > mm/khugepaged.c | 11 +++-------- > 2 files changed, 4 insertions(+), 9 deletions(-) > > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h > index 2305df6cb485..f2472c1c132a 100644 > --- a/include/trace/events/huge_memory.h > +++ b/include/trace/events/huge_memory.h > @@ -19,7 +19,7 @@ > EM( SCAN_PTE_NON_PRESENT, "pte_non_present") \ > EM( SCAN_PTE_UFFD_WP, "pte_uffd_wp") \ > EM( SCAN_PTE_MAPPED_HUGEPAGE, "pte_mapped_hugepage") \ > - EM( SCAN_PAGE_RO, "no_writable_page") \ > + EM( SCAN_PAGE_RO, "no_writable_page") /* deprecated */ \ > EM( SCAN_LACK_REFERENCED_PAGE, "lack_referenced_page") \ > EM( SCAN_PAGE_NULL, "page_null") \ > EM( SCAN_SCAN_ABORT, "scan_aborted") \ > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 4ec324a4c1fe..5ef8482597a9 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -39,7 +39,7 @@ enum scan_result { > SCAN_PTE_NON_PRESENT, > SCAN_PTE_UFFD_WP, > SCAN_PTE_MAPPED_HUGEPAGE, > - SCAN_PAGE_RO, > + SCAN_PAGE_RO, /* deprecated */ Why can't we remove that completely. -- Cheers David / dhildenb