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Documentation: cgroup: add memory.swap.{current,max} description

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The rationale of separate swap counter is given by Johannes Weiner.

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Vladimir Davydov 2016-01-20 15:03:13 -08:00 committed by Linus Torvalds
parent 5ccc5abaaf
commit 3e24b19dd5
1 changed files with 33 additions and 0 deletions
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33
Documentation/cgroup-v2.txt
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@ -819,6 +819,22 @@ PAGE_SIZE multiple when read back.
the cgroup. This may not exactly match the number of
processes killed but should generally be close.
memory.swap.current
A read-only single value file which exists on non-root
cgroups.
The total amount of swap currently being used by the cgroup
and its descendants.
memory.swap.max
A read-write single value file which exists on non-root
cgroups. The default is "max".
Swap usage hard limit. If a cgroup's swap usage reaches this
limit, anonymous meomry of the cgroup will not be swapped out.
5-2-2. General Usage
@ -1291,3 +1307,20 @@ allocation from the slack available in other groups or the rest of the
system than killing the group. Otherwise, memory.max is there to
limit this type of spillover and ultimately contain buggy or even
malicious applications.
The combined memory+swap accounting and limiting is replaced by real
control over swap space.
The main argument for a combined memory+swap facility in the original
cgroup design was that global or parental pressure would always be
able to swap all anonymous memory of a child group, regardless of the
child's own (possibly untrusted) configuration. However, untrusted
groups can sabotage swapping by other means - such as referencing its
anonymous memory in a tight loop - and an admin can not assume full
swappability when overcommitting untrusted jobs.
For trusted jobs, on the other hand, a combined counter is not an
intuitive userspace interface, and it flies in the face of the idea
that cgroup controllers should account and limit specific physical
resources. Swap space is a resource like all others in the system,
and that's why unified hierarchy allows distributing it separately.