Re: [PATCH] mm/mempolicy: Fix decision-making issues for memory migration during NUMA balancing

[Gregory Price <gourry@gourry.net>]

On Mon, Nov 25, 2024 at 12:33:52PM +0100, Michal Hocko wrote:
> On Sun 24-11-24 03:09:35, Junjie Fu wrote:
> > When handling a page fault caused by NUMA balancing (do_numa_page), it is
> > necessary to decide whether to migrate the current page to another node or
> > keep it on its current node. For pages with the MPOL_PREFERRED memory
> > policy, it is sufficient to check whether the first node set in the
> > nodemask is the same as the node where the page is currently located. If
> > this is the case, the page should remain in its current state. Otherwise,
> > migration to another node should be attempted.
> > 
> > Because the definition of MPOL_PREFERRED is as follows: "This mode sets the
> > preferred node for allocation. The kernel will try to allocate pages from
> > this node first and fall back to nearby nodes if the preferred node is low
> > on free memory. If the nodemask specifies more than one node ID, the first
> > node in the mask will be selected as the preferred node."
> > 
> > Thus, if the node where the current page resides is not the first node in
> > the nodemask, it is not the PREFERRED node, and memory migration can be
> > attempted.
> > 
> > However, in the original code, the check only verifies whether the current
> > node exists in the nodemask (which may or may not be the first node in the
> > mask). This could lead to a scenario where, if the current node is not the
> > first node in the nodemask, the code incorrectly decides not to attempt
> > migration to other nodes.
> > 
> > This behavior is clearly incorrect. If the target node for migration and
> > the page's current NUMA node are both within the nodemask but neither is
> > the first node, they should be treated with the same priority, and
> > migration attempts should proceed.
> 
> The code is clearly confusing but is there any actual problem to be
> solved? IIRC although we do keep nodemask for MPOL_PREFERRED
> policy we do not allow to set more than a single node to be set there.
> Have a look at mpol_new_preferred
>

concur here - the proposed patch doesn't actually change any behavior
(or it shouldn, at least).

Is there a migration error being observed that this patch fixes, or is
this just an `observational fix`?

~Gregory