Copy-on-write: Difference between revisions

Revision as of 10:52, 30 July 2022

Revision as of 01:16, 1 August 2022

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Copy-on-write pages are also used in the [[Linux kernel]]'s [[Kernel same-page merging|same-page merging]] feature.<ref>{{cite web|last=Abbas|first=Ali|title=The Kernel Samepage Merging Process|url=http://alouche.net/blog/2011/07/18/the-kernel-samepage-merging-process/|website=alouche.net|access-date=4 August 2016|archive-url=https://web.archive.org/web/20160808174912/http://alouche.net/blog/2011/07/18/the-kernel-samepage-merging-process/|archive-date=8 August 2016 }}</ref>

Loading the libraries for an application is also a use of copy-on-write technique. The dynamic linker maps libraries as private like follows. Any writing action on the libraries will trigger a COW in virtual memory management.

<syntaxhighlight lang="c">

open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3

mmap(NULL, 3906144, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0)

mmap(0x7f8a3ced4000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b0000)

</syntaxhighlight>

==In software==