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SubjectRe: [PATCH] doc: cgroup: update note about conditions when oom killer is invoked
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On 11/05/2020 11.39, Michal Hocko wrote:
> On Fri 08-05-20 17:16:29, Konstantin Khlebnikov wrote:
>> Starting from v4.19 commit 29ef680ae7c2 ("memcg, oom: move out_of_memory
>> back to the charge path") cgroup oom killer is no longer invoked only from
>> page faults. Now it implements the same semantics as global OOM killer:
>> allocation context invokes OOM killer and keeps retrying until success.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
>> ---
>> Documentation/admin-guide/cgroup-v2.rst | 17 ++++++++---------
>> 1 file changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>> index bcc80269bb6a..1bb9a8f6ebe1 100644
>> --- a/Documentation/admin-guide/cgroup-v2.rst
>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>> @@ -1172,6 +1172,13 @@ PAGE_SIZE multiple when read back.
>> Under certain circumstances, the usage may go over the limit
>> temporarily.
>>
>> + In default configuration regular 0-order allocation always
>> + succeed unless OOM killer choose current task as a victim.
>> +
>> + Some kinds of allocations don't invoke the OOM killer.
>> + Caller could retry them differently, return into userspace
>> + as -ENOMEM or silently ignore in cases like disk readahead.
>
> I would probably add -EFAULT but the less error codes we document the
> better.

Yeah, EFAULT was a most obscure result of memory shortage.
Fortunately with new behaviour this shouldn't happens a lot.

Actually where it is still possible? THP always fallback to 0-order.
I mean EFAULT could appear inside kernel only if task is killed so
nobody would see it.

>
>> +
>> This is the ultimate protection mechanism. As long as the
>> high limit is used and monitored properly, this limit's
>> utility is limited to providing the final safety net.
>> @@ -1228,17 +1235,9 @@ PAGE_SIZE multiple when read back.
>> The number of time the cgroup's memory usage was
>> reached the limit and allocation was about to fail.
>>
>> - Depending on context result could be invocation of OOM
>> - killer and retrying allocation or failing allocation.
>> -
>> - Failed allocation in its turn could be returned into
>> - userspace as -ENOMEM or silently ignored in cases like
>> - disk readahead. For now OOM in memory cgroup kills
>> - tasks iff shortage has happened inside page fault.
>> -
>> This event is not raised if the OOM killer is not
>> considered as an option, e.g. for failed high-order
>> - allocations.
>> + allocations or if caller asked to not retry attempts.
>>
>> oom_kill
>> The number of processes belonging to this cgroup
>

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