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

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Date:	Wed, 26 Aug 2015 18:24:34 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/12] mm, page_alloc: Distinguish between being unable to
 sleep, unwilling to sleep and avoiding waking kswapd

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.

Thanks.

>>> @@ -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)))
>>>   			filter &= ~SHOW_MEM_FILTER_NODES;
>>> -	if (in_interrupt() || !(gfp_mask & __GFP_WAIT))
>>> +	if (in_interrupt() || !(gfp_mask & __GFP_WAIT) || (gfp_mask & __GFP_ATOMIC))
>>>   		filter &= ~SHOW_MEM_FILTER_NODES;
>>>
>>>   	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
> time.

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
> cases.

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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