linux-kernel - Re: [PATCH v2 3/3] mm/mempolicy: introduce MPOL_WEIGHTED

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Message-ID: <ZbFQgSFfqDF+UvSX@memverge.com>
Date: Wed, 24 Jan 2024 13:01:37 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	corbet@....net, akpm@...ux-foundation.org, honggyu.kim@...com,
	rakie.kim@...com, hyeongtak.ji@...com, mhocko@...nel.org,
	vtavarespetr@...ron.com, jgroves@...ron.com,
	ravis.opensrc@...ron.com, sthanneeru@...ron.com,
	emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
	hannes@...xchg.org, dan.j.williams@...el.com,
	Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v2 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
 for weighted interleaving

On Wed, Jan 24, 2024 at 09:51:20AM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@...verge.com> writes:
> 
> +	if (new && (new->mode == MPOL_INTERLEAVE ||
> +		    new->mode == MPOL_WEIGHTED_INTERLEAVE))
>  		current->il_prev = MAX_NUMNODES-1;
>  	task_unlock(current);
>  	mpol_put(old);
> 
> I don't think we need to change this.
>

Ah you're right it's set to MAX_NUMNODES-1 here, but NUMA_NO_NODE can be
passed in as an argument to alloc_pages_bulk_array_mempolicy, like here:

vm_area_alloc_pages()
	if (IS_ENABLED(CONFIG_NUMA) && nid == NUMA_NO_NODE)
		nr = alloc_pages_bulk_array_mempolicy(bulk_gfp,
			nr_pages_request,
			pages + nr_allocated);

> > (cur_weight = 0) can happen in two scenarios:
> >   - initial setting of mempolicy (NUMA_NO_NODE w/ cur_weight=0)
> >   - weighted_interleave_nodes decrements it down to 0
> >
> > Now that i'm looking at it - the second condition should not exist, and
> > we can eliminate it. The logic in weighted_interleave_nodes is actually
> > annoyingly unclear at the moment, so I'm going to re-factor it a bit to
> > be more explicit.
> 
> I am OK with either way.  Just a reminder, the first condition may be
> true in alloc_pages_bulk_array_weighted_interleave() and perhaps some
> other places.
> 

Yeah, the bulk allocator handles it correctly, it's just a matter of
clarity for weighted_interleave_nodes.

What isn't necessarily handled correctly is the rebind code. Rebind due
to a cgroup/mems_allowed change can cause a stale weight to be carried.

Basically cur_weight is not cleared, but the node it applied to may no
longer be the next node when next_node_in() is called.

The race condition is 1) exceedingly rare, and 2) not necessarily harmful,
just inaccurate. The worst case scenario is that a node receives up to 255
additional allocations once after a rebind (but more likely 10-20).

I was considering forcing the interleave forward like this:

@@ -356,6 +361,10 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
                tmp = *nodes;

        pol->nodes = tmp;
+
+       /* Weighted interleave policies are forced forward to the next node */
+       if (pol->mode & MPOL_WEIGHTED_INTERLEAVE)
+               pol->wil.cur_weight = 0;
 }

But this creates 2 race conditions when we read cur_weight and nodemask
in the allocator path.

Example 1:
1) bulk allocator READ_ONCE(mask), READ_ONCE(cur_weight)
2) rebind changes nodemask and { cur_weight = 0; }
3) bulk allocator sets pol->wil.cur_weight

In this scenario, resume_weight is stale coming out of bulk allocations
if the resume_node has been removed from the node mask.

Example 2:
1) rebind changes nodemask
2) bulk allocator READ_ONCE(mask), READ_ONCE(cur_weight)
3) rebind sets { cur_weight = 0; }

In this scenario, cur_weight is stale going into bulk allocations.

Neither of these can force a violation of mems_allowed, just a
mis-application of a weight.

I'll need to think on this a bit.  We can either leave this as-is,
meaning the first allocation after a rebind may apply the wrong weight
to a node, or we can try to track the current-interleave-node and
validate next_node_in(mask) == current-interleave-node before leaving
the allocator path (this may also be just as racey).

turns out concurrent counting is still hard :]

~Gregory