Re: [PATCH] doc: cgroup: update note about conditions when oom killer is invoked

[Konstantin Khlebnikov]

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@xxxxxxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> > ---
> >   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