linux-kernel - Re: [PATCH 2/2] mm: do not loop over ALLOC_NO

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Date:	Wed, 18 Nov 2015 16:19:14 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without
 triggering reclaim

On 11/18/2015 04:11 PM, Michal Hocko wrote:
> On Wed 18-11-15 15:57:45, Vlastimil Babka wrote:
> [...]
>> > --- a/mm/page_alloc.c
>> > +++ b/mm/page_alloc.c
>> > @@ -3046,32 +3046,36 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>> >  		 * allocations are system rather than user orientated
>> >  		 */
>> >  		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
>> > -		do {
>> > -			page = get_page_from_freelist(gfp_mask, order,
>> > -							ALLOC_NO_WATERMARKS, ac);
>> > -			if (page)
>> > -				goto got_pg;
>> > -
>> > -			if (gfp_mask & __GFP_NOFAIL)
>> > -				wait_iff_congested(ac->preferred_zone,
>> > -						   BLK_RW_ASYNC, HZ/50);
>> 
>> I've been thinking if the lack of unconditional wait_iff_congested() can affect
>> something negatively. I guess not?
> 
> Considering that the wait_iff_congested is removed only for PF_MEMALLOC
> with __GFP_NOFAIL which should be non-existent in the kernel then I

Hm that one won't reach it indeed, but also not loop, so that wasn't my concern.
I was referring to:

        /* Keep reclaiming pages as long as there is reasonable progress */                            
        pages_reclaimed += did_some_progress;
        if ((did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) ||
            ((gfp_mask & __GFP_REPEAT) && pages_reclaimed < (1 << order))) {                           
                /* Wait for some write requests to complete then retry */
                wait_iff_congested(ac->preferred_zone, BLK_RW_ASYNC, HZ/50);                           
                goto retry;                                                                            
        }

Here we might skip the wait_iff_congested and go straight for oom. But it's true
that ordinary allocations that fail to make progress will also not wait, so I
guess it's fine.

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> think the risk is really low. Even if there was a caller _and_ there
> was a congestion then the behavior wouldn't be much more worse than
> what we have currently. The system is out of memory hoplessly if
> ALLOC_NO_WATERMARKS allocation fails.
> 

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