Re: [PATCH] mm, page_alloc: actually ignore mempolicies for high priority allocations

[Michal Hocko <mhocko@kernel.org>]

On Wed 15-08-18 15:16:52, Andrew Morton wrote:
[...]
> From: Vlastimil Babka <vbabka@suse.cz>
> Subject: mm, page_alloc: actually ignore mempolicies for high priority allocations
> 
> The __alloc_pages_slowpath() function has for a long time contained code
> to ignore node restrictions from memory policies for high priority
> allocations.  The current code that resets the zonelist iterator however
> does effectively nothing after commit 7810e6781e0f ("mm, page_alloc: do
> not break __GFP_THISNODE by zonelist reset") removed a buggy zonelist
> reset.  Even before that commit, mempolicy restrictions were still not
> ignored, as they are passed in ac->nodemask which is untouched by the
> code.
> 
> We can either remove the code, or make it work as intended.  Since
> ac->nodemask can be set from task's mempolicy via alloc_pages_current()
> and thus also alloc_pages(), it may indeed affect kernel allocations, and
> it makes sense to ignore it to allow progress for high priority
> allocations.
> 
> Thus, this patch resets ac->nodemask to NULL in such cases.  This assumes
> all callers can handle it (i.e.  there are no guarantees as in the case of
> __GFP_THISNODE) which seems to be the case.  The same assumption is
> already present in check_retry_cpuset() for some time.
> 
> The expected effect is that high priority kernel allocations in the
> context of userspace tasks (e.g.  OOM victims) restricted by mempolicies
> will have higher chance to succeed if they are restricted to nodes with
> depleted memory, while there are other nodes with free memory left.
> 
> 
> Ot's not a new intention, but for the first time the code will match the
> intention, AFAICS.  It was intended by commit 183f6371aac2 ("mm: ignore
> mempolicies when using ALLOC_NO_WATERMARK") in v3.6 but I think it never
> really worked, as mempolicy restriction was already encoded in nodemask,
> not zonelist, at that time.
> 
> So originally that was for ALLOC_NO_WATERMARK only.  Then it was adjusted
> by e46e7b77c909 ("mm, page_alloc: recalculate the preferred zoneref if the
> context can ignore memory policies") and cd04ae1e2dc8 ("mm, oom: do not
> rely on TIF_MEMDIE for memory reserves access") to the current state.  So
> even GFP_ATOMIC would now ignore mempolicies after the initial attempts
> fail - if the code worked as people thought it does.
> 
> Link: http://lkml.kernel.org/r/20180612122624.8045-1-vbabka@suse.cz
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

The code is quite subtle and we have a bad history of copying stuff
without rethinking whether the code still is needed. Which is sad and a
clear sign that the code is too complex. I cannot say this change
doesn't have any subtle side effects but it makes the intention clear at
least so I _think_ it is good to go. If we find some unintended side
effects we should simply rethink the whole reset zonelist thing.

That being said
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 
>  mm/page_alloc.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> --- a/mm/page_alloc.c~mm-page_alloc-actually-ignore-mempolicies-for-high-priority-allocations
> +++ a/mm/page_alloc.c
> @@ -4165,11 +4165,12 @@ retry:
>  		alloc_flags = reserve_flags;
>  
>  	/*
> -	 * Reset the zonelist iterators if memory policies can be ignored.
> -	 * These allocations are high priority and system rather than user
> -	 * orientated.
> +	 * Reset the nodemask and zonelist iterators if memory policies can be
> +	 * ignored. These allocations are high priority and system rather than
> +	 * user oriented.
>  	 */
>  	if (!(alloc_flags & ALLOC_CPUSET) || reserve_flags) {
> +		ac->nodemask = NULL;
>  		ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
>  					ac->high_zoneidx, ac->nodemask);
>  	}
> _
> 

-- 
Michal Hocko
SUSE Labs