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Message-ID: <ZbPf6d2cQykdl3Eb@memverge.com>
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,
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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
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