linux-kernel - Re: [PATCH v3 4/4] mm/mempolicy: change cur_il_weight to atomic and carry the node with it

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Date: Fri, 26 Jan 2024 11:38:01 -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
Subject: Re: [PATCH v3 4/4] mm/mempolicy: change cur_il_weight to atomic and
 carry the node with it

On Fri, Jan 26, 2024 at 03:40:27PM +0800, Huang, Ying wrote:
> Gregory Price <gourry.memverge@...il.com> writes:
> 
> > Two special observations:
> > - if the weight is non-zero, cur_il_weight must *always* have a
> >   valid node number, e.g. it cannot be NUMA_NO_NODE (-1).
> 
> IIUC, we don't need that, "MAX_NUMNODES-1" is used instead.
> 

Correct, I just thought it pertinent to call this out explicitly since
I'm stealing the top byte, but the node value has traditionally been a
full integer.

This may be relevant should anyone try to carry, a random node value
into this field. For example, if someone tried to copy policy->home_node
into cur_il_weight for whatever reason.

It's worth breaking out a function to defend against this - plus to hide
the bit operations directly as you recommend below.

> >  	/* Weighted interleave settings */
> > -	u8 cur_il_weight;
> > +	atomic_t cur_il_weight;
> 
> If we use this field for node and weight, why not change the field name?
> For example, cur_wil_node_weight.
> 

ack.

> > +			if (cweight & 0xFF)
> > +				*policy = cweight >> 8;
> 
> Please define some helper functions or macros instead of operate on bits
> directly.
> 

ack.

> >  			else
> >  				*policy = next_node_in(current->il_prev,
> >  						       pol->nodes);
> 
> If we record current node in pol->cur_il_weight, why do we still need
> curren->il_prev.  Can we only use pol->cur_il_weight?  And if so, we can
> even make current->il_prev a union.
> 

I just realized that there's a problem here for shared memory policies.

from weighted_interleave_nodes, I do this:

cur_weight = atomic_read(&policy->cur_il_weight);
..
weight--;
..
atomic_set(&policy->cur_il_weight, cur_weight);

On a shared memory policy, this is a race condition.

I don't think we can combine il_prev and cur_wil_node_weight because
the task policy may be different than the current policy.

i.e. it's totally valid to do the following:

1) set_mempolicy(MPOL_INTERLEAVE)
2) mbind(..., MPOL_WEIGHTED_INTERLEAVE)

Using current->il_prev between these two policies, is just plain incorrect,
so I will need to rethink this, and the existing code will need to be
updated such that weighted_interleave does not use current->il_prev.

~Gregory