Re: [PATCH 0/6] Optimise page alloc/free fast paths followup v2

[Vlastimil Babka]

On 05/03/2016 10:50 AM, Mel Gorman wrote:
> On Mon, May 02, 2016 at 10:54:23AM +0200, Vlastimil Babka wrote:
> > On 04/27/2016 04:57 PM, Mel Gorman wrote:
> > > as the patch "mm, page_alloc: inline the fast path of the zonelist iterator"
> > > is fine. The nodemask pointer is the same between cpuset retries. If the
> > > zonelist changes due to ALLOC_NO_WATERMARKS *and* it races with a cpuset
> > > change then there is a second harmless pass through the page allocator.
> >
> > True. But I just realized (while working on direct compaction priorities)
> > that there's another subtle issue with the ALLOC_NO_WATERMARKS part.
> > According to the comment it should be ignoring mempolicies, but it still
> > honours ac.nodemask, and your patch is replacing NULL ac.nodemask with the
> > mempolicy one.
> >
> > I think it's possibly easily fixed outside the fast path like this. If
> > you agree, consider it has my s-o-b:
>
> While I see your point, I don't necessarily see why this fixes it as the
> original nodemask may also be a restricted set that ALLOC_NO_WATERMARKS
> should ignore.

I wasn't so sure about that, so I defensively went with just restoring 
the pre-patch behavior. I expect that it's safe to ignore mempolicies 
imposed on the process via e.g. taskset/numactl, for a a critical kernel 
allocation that happens to be done within the process context. But 
ignoring nodemask that's imposed by the kernel allocation itself might 
be breaking some expectations. I guess vma policies can also result in a 
restricted nodemask, but those should be for userspace allocations and 
never ALLOC_NO_WATERMARKS?

> How about this?
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 79100583b9de..dbb08d102d41 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3432,9 +3432,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>   		/*
>   		 * Ignore mempolicies if ALLOC_NO_WATERMARKS on the grounds
>   		 * the allocation is high priority and these type of
> -		 * allocations are system rather than user orientated
> +		 * allocations are system rather than user orientated. If a
> +		 * cpuset retry occurs then these values persist across the
> +		 * retry but that's ok for a context ignoring watermarks.
>   		 */
>   		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> +		ac->high_zoneidx = MAX_NR_ZONES - 1;

Wouldn't altering high_zoneidx like this result in e.g. DMA allocation 
ending up in a NORMAL zone? Also high_zoneidx doesn't seem to get 
restricted by mempolicy/nodemask anyway. Maybe you wanted to re-set 
preferred_zoneref? That would make sense, yeah.

> +		ac->nodemask = NULL;
>   		page = get_page_from_freelist(gfp_mask, order,
>   						ALLOC_NO_WATERMARKS, ac);
>   		if (page)