On Mon, 23 Nov 2015, Michal Hocko wrote:
> > >>>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
> >
> > Ah, I see, I misinterpreted what the return values of out_of_memory() mean.
> > But now that I look at its code, it seems to only return false when
> > oom_killer_disabled is set to true. Which is a global thing and nothing to
> > do with the context of the __GFP_NOFAIL allocation?
>
> I am not sure I follow you here. The point of the warning is to warn
> when the oom killer is disbaled (out_of_memory returns false) _and_ the
> request is __GFP_NOFAIL because we simply cannot guarantee any forward
> progress and just a use of the allocation flag is not supproted.
>
I don't think the WARN_ONCE() above is helpful for a few reasons:
- it suggests that min_free_kbytes is the best way to work around such
issues and gives kernel developers a free pass to just say "raise
min_free_kbytes" rather than reducing their reliance on __GFP_NOFAIL,
- raising min_free_kbytes is not immediately actionable without memory
freeing to fix any oom issue, and
- it relies on the earlier warning to dump the state of memory and
doesn't add any significant information to help understand how seperate
occurrences are similar or different.
I think the WARN_ONCE() should just be removed.
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