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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