Re: [PATCH] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[Michal Hocko]

On Sun 22-11-15 13:55:31, Vlastimil Babka wrote:
> On 11.11.2015 14:48, mhocko@xxxxxxxxxx wrote:
> >  mm/page_alloc.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 8034909faad2..d30bce9d7ac8 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2766,8 +2766,16 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> >  			goto out;
> >  	}
> >  	/* Exhausted what can be done so it's blamo time */
> > -	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
> > +	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> >  		*did_some_progress = 1;
> > +
> > +		if (gfp_mask & __GFP_NOFAIL) {
> > +			page = get_page_from_freelist(gfp_mask, order,
> > +					ALLOC_NO_WATERMARKS|ALLOC_CPUSET, ac);
> > +			WARN_ONCE(!page, "Unable to fullfil gfp_nofail allocation."
> > +				    " Consider increasing min_free_kbytes.\n");
> 
> It seems redundant to me to keep the WARN_ON_ONCE also above in the if () part?

They are warning about two different things. The first one catches a
buggy code which uses __GFP_NOFAIL from oom disabled context while the
second one tries to help the administrator with a hint that memory
reserves are too small.

> Also s/gfp_nofail/GFP_NOFAIL/ for consistency?

Fair enough, changed.

> Hm and probably out of scope of your patch, but I understand the WARN_ONCE
> (WARN_ON_ONCE) to be _ONCE just to prevent a flood from a single task looping
> here. But for distinct tasks and potentially far away in time, wouldn't we want
> to see all the warnings? Would that be feasible to implement?

I was thinking about that as well some time ago but it was quite
hard to find a good enough API to tell when to warn again. The first
WARN_ON_ONCE should trigger for all different _code paths_ no matter
how frequently they appear to catch all the buggy callers. The second
one would benefit from a new warning after min_free_kbytes was updated
because it would tell the administrator that the last update was not
sufficient for the workload.

> 
> > +		}
> > +	}
> >  out:
> >  	mutex_unlock(&oom_lock);
> >  	return page;
> > 

Thanks!
-- 
Michal Hocko
SUSE Labs
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