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cgroup: add documentation on extended attributes usage
v2: update cgroups.txt instead of creating a new file Cc: Tejun Heo <tj@kernel.org> Cc: Hugh Dickins <hughd@google.com> Cc: Hillf Danton <dhillf@gmail.com> Cc: Lennart Poettering <lpoetter@redhat.com> Acked-by: Li Zefan <lizefan@huawei.com> Signed-off-by: Aristeu Rozanski <aris@redhat.com> Signed-off-by: Tejun Heo <tj@kernel.org>
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@ -29,7 +29,8 @@ CONTENTS:
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3.1 Overview
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3.2 Synchronization
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3.3 Subsystem API
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4. Questions
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4. Extended attributes usage
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5. Questions
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1. Control Groups
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=================
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@ -650,7 +651,26 @@ and root cgroup. Currently this will only involve movement between
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the default hierarchy (which never has sub-cgroups) and a hierarchy
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that is being created/destroyed (and hence has no sub-cgroups).
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4. Questions
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4. Extended attribute usage
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===========================
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cgroup filesystem supports certain types of extended attributes in its
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directories and files. The current supported types are:
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- Trusted (XATTR_TRUSTED)
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- Security (XATTR_SECURITY)
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Both require CAP_SYS_ADMIN capability to set.
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Like in tmpfs, the extended attributes in cgroup filesystem are stored
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using kernel memory and it's advised to keep the usage at minimum. This
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is the reason why user defined extended attributes are not supported, since
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any user can do it and there's no limit in the value size.
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The current known users for this feature are SELinux to limit cgroup usage
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in containers and systemd for assorted meta data like main PID in a cgroup
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(systemd creates a cgroup per service).
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5. Questions
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============
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Q: what's up with this '/bin/echo' ?
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