Messages in this thread | | | Date | Mon, 23 Nov 2015 13:26:49 -0800 (PST) | From | David Rientjes <> | Subject | Re: [PATCH] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves |
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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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