Re: [patch 2/9] mm: memcontrol: rearrange charging fast path

[Johannes Weiner]

On Wed, May 07, 2014 at 04:33:34PM +0200, Michal Hocko wrote:
> On Wed 30-04-14 16:25:36, Johannes Weiner wrote:
> > The charging path currently starts out with OOM condition checks when
> > OOM is the rarest possible case.
> > 
> > Rearrange this code to run OOM/task dying checks only after trying the
> > percpu charge and the res_counter charge and bail out before entering
> > reclaim.  Attempting a charge does not hurt an (oom-)killed task as
> > much as every charge attempt having to check OOM conditions. 
> 
> OK, I've never considered those to be measurable but it is true that the
> numbers accumulate over time.
> 
> So yes, this makes sense.
> 
> > Also, only check __GFP_NOFAIL when the charge would actually fail.
> 
> OK, but return ENOMEM as pointed below.
> 
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 31 ++++++++++++++++---------------
> >  1 file changed, 16 insertions(+), 15 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 75dfeb8fa98b..6ce59146fec7 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2598,21 +2598,6 @@ static int mem_cgroup_try_charge(struct mem_cgroup *memcg,
> >  
> >  	if (mem_cgroup_is_root(memcg))
> >  		goto done;
> > -	/*
> > -	 * Unlike in global OOM situations, memcg is not in a physical
> > -	 * memory shortage.  Allow dying and OOM-killed tasks to
> > -	 * bypass the last charges so that they can exit quickly and
> > -	 * free their memory.
> > -	 */
> > -	if (unlikely(test_thread_flag(TIF_MEMDIE) ||
> > -		     fatal_signal_pending(current)))
> > -		goto bypass;
> 
> This is missing "memcg: do not hang on OOM when killed by userspace OOM
> access to memory reserves" - trivial to resolve.

Yep, will rebase before the next submission.

> > -	if (unlikely(task_in_memcg_oom(current)))
> > -		goto nomem;
> > -
> > -	if (gfp_mask & __GFP_NOFAIL)
> > -		oom = false;
> >  retry:
> >  	if (consume_stock(memcg, nr_pages))
> >  		goto done;
> [...]
> > @@ -2662,6 +2660,9 @@ retry:
> >  	if (mem_cgroup_wait_acct_move(mem_over_limit))
> >  		goto retry;
> >  
> > +	if (gfp_mask & __GFP_NOFAIL)
> > +		goto bypass;
> > +
> 
> This is a behavior change because we have returned ENOMEM previously

__GFP_NOFAIL must never return -ENOMEM, or we'd have to rename it ;-)
It just looks like this in the patch, but this is the label code:

nomem:
	if (!(gfp_mask & __GFP_NOFAIL))
		return -ENOMEM;
bypass:
	...
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