Re: [PATCH 07/12] mm, page_alloc: Distinguish between being unable to sleep, unwilling to sleep and avoiding waking kswapd

From: Vlastimil Babka
Date: Wed Aug 26 2015 - 12:25:28 EST

On 08/26/2015 04:45 PM, Mel Gorman wrote:
On Tue, Aug 25, 2015 at 05:37:59PM +0200, Vlastimil Babka wrote:
@@ -2158,7 +2158,7 @@ static bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
return false;
if (fail_page_alloc.ignore_gfp_highmem && (gfp_mask & __GFP_HIGHMEM))
return false;
- if (fail_page_alloc.ignore_gfp_wait && (gfp_mask & __GFP_WAIT))
+ if (fail_page_alloc.ignore_gfp_wait && (gfp_mask & (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
return false;

return should_fail(&fail_page_alloc.attr, 1 << order);

IIUC ignore_gfp_wait tells it to assume that reclaimers will eventually
succeed (for some reason?), so they shouldn't fail. Probably to focus the
testing on atomic allocations. But your change makes atomic allocation never
fail, so that goes against the knob IMHO?

Fair point, I'll remove the __GFP_ATOMIC check. I felt this was a sensible
but then again deliberately failing allocations makes my brain twitch a
bit. In retrospect, someone who cared should add a ignore_gfp_atomic knob.


@@ -2660,7 +2660,7 @@ void warn_alloc_failed(gfp_t gfp_mask, int order, const char *fmt, ...)
if (test_thread_flag(TIF_MEMDIE) ||
(current->flags & (PF_MEMALLOC | PF_EXITING)))
- if (in_interrupt() || !(gfp_mask & __GFP_WAIT))
+ if (in_interrupt() || !(gfp_mask & __GFP_WAIT) || (gfp_mask & __GFP_ATOMIC))

if (fmt) {

This caught me previously and I convinced myself that it's OK, but now I'm
not anymore. IIUC this is to not filter nodes by mems_allowed during
printing, if the allocation itself wasn't limited? In that case it should
probably only look at __GFP_ATOMIC after this patch? As that's the only
thing that determines ALLOC_CPUSET.
I don't know where in_interrupt() comes from, but it was probably considered
in the past, as can be seen in zlc_setup()?

I assumed the in_interrupt() thing was simply because cpusets were the
primary means of limiting allocations of interest to the author at the

IIUC this hunk is unrelated to the previous one - not about limiting allocations, but printing allocation warnings. Which includes the state of nodes where the allocation was allowed to try. And ~SHOW_MEM_FILTER_NODES means it was allowed everywhere, so the printing won't filter by mems_allowed.

I guess now that I think about it more that a more sensible check would
be against __GFP_DIRECT_RECLAIM because that covers the interesting

I think the most robust check would be to rely on what was already prepared by gfp_to_alloc_flags(), instead of repeating it here. So add alloc_flags parameter to warn_alloc_failed(), and drop the filter when
- ALLOC_CPUSET is not set, as that disables the cpuset checks
- ALLOC_NO_WATERMARKS is set, as that allows calling
__alloc_pages_high_priority() attempt which ignores cpusets

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