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Kernel same-page merging
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<h2 id="deduplication">Deduplication</h2> <p>KSM performs memory deduplication by scanning through <a href="/facts/Main_memory/wPHKFpdx">main memory</a> for physical pages that have identical content, and identifies the virtual pages that are mapped to those physical pages. It leaves one page unchanged, and re-maps each duplicate page to point to the same physical page, after which it releases the extra physical pages for re-use. It also marks both virtual pages as "<a href="/facts/Copy-on-write/Vz5V8uMX">copy-on-write</a>" (COW), so that kernel will automatically remap a virtual page back to having its own separate physical page as soon as any process begins to write to it.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p><p>KSM was originally intended to run more virtual machines on one host by <a href="/facts/Shared_memory/zmpH7WuV">sharing memory</a> between processes as well as virtual machines. An experimental implementation of KSM by Red Hat found that 52 virtual instances of Windows XP with 1 GB of memory, could run on a host computer that had only 16 GB of RAM.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> Upon its <a href="/facts/Implementation/IH1KXgP3">implementation</a>, users found KSM to also be useful for non-virtualized environments in which memory is at a premium.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p><p>KSM was merged into the <a href="/facts/Linux_kernel/dlkduHuA">Linux kernel</a> mainline in version 2.6.32, which was released on December 3, 2009.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> To be effective, the operating system kernel must find identical memory pages held by different processes. The kernel also needs to predict whether the pages are likely to update infrequently enough that the merging would be an efficient use of processor resources.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> A concern is that although memory usage is reduced, CPU usage is increased, thus negating potential increases in performance.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p> <h2 id="security-risks">Security risks</h2> <p>Security is also a concern: </p> <ul><li>Allows circumvention of <a href="/facts/Address_space_layout_randomization/K6lMxWTr">address space layout randomization</a> (ASLR)<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></li> <li>Exposes information via <a href="/facts/Timing_attack/Aau4qNoh">timing attacks</a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a></li> <li>Allows contamination of cryptographic resources in other virtualized guests via the memory <a href="/facts/Row_hammer/XCaHA39b">row hammer</a> attack<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a></li></ul> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Data_deduplication/sHuJ8zya">Data deduplication</a> – Reduction of storage requirement by sharing duplicate files</li> <li><a href="/facts/Single-instance_storage/muPa4Bfh">Single-instance storage</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://www.kernel.org/doc/Documentation/vm/ksm.txt">Linux kernel documentation</a></li> <li><a href="https://fedoraproject.org/wiki/Features/KSM">Fedora KSM page</a></li> <li><a href="http://www.linux-kvm.org/page/KSM">Kernel-based virtual machine site - KSM</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"KSM tries again". lwn.net. kernel.org. Retrieved August 21, 2010. <a href="https://lwn.net/Articles/330589/" target="_blank">https://lwn.net/Articles/330589/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Linux kernel 2.6.32, Section 1.3. Kernel Samepage Merging (memory deduplication)". KernelNewbies.org. December 3, 2009. Retrieved August 12, 2015. <a href="https://kernelnewbies.org/Linux_2_6_32#Kernel_Samepage_Merging_.28memory_deduplication.29" target="_blank">https://kernelnewbies.org/Linux_2_6_32#Kernel_Samepage_Merging_.28memory_deduplication.29</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Anatomy of Linux Kernel Shared Memory". IBM Developer. IBM. Archived from the original on July 9, 2016. Retrieved August 21, 2010. <a href="https://web.archive.org/web/20160709062356/http://www.ibm.com/developerworks/linux/library/l-kernel-shared-memory/" target="_blank">https://web.archive.org/web/20160709062356/http://www.ibm.com/developerworks/linux/library/l-kernel-shared-memory/</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Increasing memory density by using KSM" (PDF). Kernel.org. Retrieved August 21, 2010. <a href="https://www.kernel.org/doc/ols/2009/ols2009-pages-19-28.pdf" target="_blank">https://www.kernel.org/doc/ols/2009/ols2009-pages-19-28.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Anatomy of Linux Kernel Shared Memory". IBM Developer. IBM. Archived from the original on July 9, 2016. Retrieved August 21, 2010. <a href="https://web.archive.org/web/20160709062356/http://www.ibm.com/developerworks/linux/library/l-kernel-shared-memory/" target="_blank">https://web.archive.org/web/20160709062356/http://www.ibm.com/developerworks/linux/library/l-kernel-shared-memory/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Linux kernel 2.6.32, Section 1.3. Kernel Samepage Merging (memory deduplication)". KernelNewbies.org. December 3, 2009. Retrieved August 12, 2015. <a href="https://kernelnewbies.org/Linux_2_6_32#Kernel_Samepage_Merging_.28memory_deduplication.29" target="_blank">https://kernelnewbies.org/Linux_2_6_32#Kernel_Samepage_Merging_.28memory_deduplication.29</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Increasing memory density by using KSM" (PDF). Kernel.org. Retrieved August 21, 2010. <a href="https://www.kernel.org/doc/ols/2009/ols2009-pages-19-28.pdf" target="_blank">https://www.kernel.org/doc/ols/2009/ols2009-pages-19-28.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"KSM tries again". lwn.net. kernel.org. Retrieved August 21, 2010. <a href="https://lwn.net/Articles/330589/" target="_blank">https://lwn.net/Articles/330589/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Kuniyasu Suzaki; Kengo Iijima; Toshiki Yagi; Cyrille Artho. "Memory Deduplication as a Threat to the Guest OS" (PDF). Archived from the original (PDF) on January 5, 2013. Retrieved September 16, 2013. <a href="https://web.archive.org/web/20130105174201/http://staff.aist.go.jp/k.suzaki/EuroSec2011-suzaki.pdf" target="_blank">https://web.archive.org/web/20130105174201/http://staff.aist.go.jp/k.suzaki/EuroSec2011-suzaki.pdf</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Barresi, Antonio; Razavi, Kaveh; Payer, Mathias; Gross, Thomas R. (August 2015). "CAIN: Silently Breaking ASLR in the Cloud" (PDF). USENIX. Retrieved August 12, 2015. <a href="/wiki/Mathias_Payer" target="_blank">/wiki/Mathias_Payer</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Red Hat; Polyakov, Andy. "It's all a question of time - AES timing attacks on OpenSSL". access.redhat.com. Red Hat. Retrieved August 4, 2016. <a href="https://access.redhat.com/blogs/766093/posts/1976303" target="_blank">https://access.redhat.com/blogs/766093/posts/1976303</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Schwarzl, Martin; Kraft, Erik; Lipp, Moritz; Gruss, Daniel (2021). "Remote Memory-Deduplication Attacks". Network and Distributed System Security Symposium. Retrieved April 2, 2022. <a href="https://graz.pure.elsevier.com/en/publications/remote-memory-deduplication-attacks" target="_blank">https://graz.pure.elsevier.com/en/publications/remote-memory-deduplication-attacks</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"New FFS Rowhammer Attack Hijacks Linux VMs". August 14, 2016. Retrieved August 17, 2016. <a href="https://news.softpedia.com/news/new-ffs-rowhammer-attack-targets-linux-vm-setups-507290.shtml" target="_blank">https://news.softpedia.com/news/new-ffs-rowhammer-attack-targets-linux-vm-setups-507290.shtml</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> </ol>