Re: [PATCH v3 2/2] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED_MANY policy

["Huang, Ying" <ying.huang@intel.com>]

Donet Tom <donettom@linux.ibm.com> writes:

> commit bda420b98505 ("numa balancing: migrate on fault among multiple bound
> nodes") added support for migrate on protnone reference with MPOL_BIND
> memory policy. This allowed numa fault migration when the executing node
> is part of the policy mask for MPOL_BIND. This patch extends migration
> support to MPOL_PREFERRED_MANY policy.
>
> Currently, we cannot specify MPOL_PREFERRED_MANY with the mempolicy flag
> MPOL_F_NUMA_BALANCING. This causes issues when we want to use
> NUMA_BALANCING_MEMORY_TIERING. To effectively use the slow memory tier,
> the kernel should not allocate pages from the slower memory tier via
> allocation control zonelist fallback. Instead, we should move cold pages
> from the faster memory node via memory demotion. For a page allocation,
> kswapd is only woken up after we try to allocate pages from all nodes in
> the allocation zone list. This implies that, without using memory
> policies, we will end up allocating hot pages in the slower memory tier.
>
> MPOL_PREFERRED_MANY was added by commit b27abaccf8e8 ("mm/mempolicy: add
> MPOL_PREFERRED_MANY for multiple preferred nodes") to allow better
> allocation control when we have memory tiers in the system. With
> MPOL_PREFERRED_MANY, the user can use a policy node mask consisting only
> of faster memory nodes. When we fail to allocate pages from the faster
> memory node, kswapd would be woken up, allowing demotion of cold pages
> to slower memory nodes.
>
> With the current kernel, such usage of memory policies implies we can't
> do page promotion from a slower memory tier to a faster memory tier
> using numa fault. This patch fixes this issue.
>
> For MPOL_PREFERRED_MANY, if the executing node is in the policy node
> mask, we allow numa migration to the executing nodes. If the executing
> node is not in the policy node mask, we do not allow numa migration.

Can we provide more information about this?  I suggest to use an
example, for instance, pages may be distributed among multiple sockets
unexpectedly.

--
Best Regards,
Huang, Ying

> Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
> Signed-off-by: Donet Tom <donettom@linux.ibm.com>
> ---
>  mm/mempolicy.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index aa48376e2d34..13100a290918 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1504,9 +1504,10 @@ static inline int sanitize_mpol_flags(int *mode, unsigned short *flags)
>  	if ((*flags & MPOL_F_STATIC_NODES) && (*flags & MPOL_F_RELATIVE_NODES))
>  		return -EINVAL;
>  	if (*flags & MPOL_F_NUMA_BALANCING) {
> -		if (*mode != MPOL_BIND)
> +		if (*mode == MPOL_BIND || *mode == MPOL_PREFERRED_MANY)
> +			*flags |= (MPOL_F_MOF | MPOL_F_MORON);
> +		else
>  			return -EINVAL;
> -		*flags |= (MPOL_F_MOF | MPOL_F_MORON);
>  	}
>  	return 0;
>  }
> @@ -2770,15 +2771,26 @@ int mpol_misplaced(struct folio *folio, struct vm_fault *vmf,
>  		break;
>  
>  	case MPOL_BIND:
> -		/* Optimize placement among multiple nodes via NUMA balancing */
> +	case MPOL_PREFERRED_MANY:
> +		/*
> +		 * Even though MPOL_PREFERRED_MANY can allocate pages outside
> +		 * policy nodemask we don't allow numa migration to nodes
> +		 * outside policy nodemask for now. This is done so that if we
> +		 * want demotion to slow memory to happen, before allocating
> +		 * from some DRAM node say 'x', we will end up using a
> +		 * MPOL_PREFERRED_MANY mask excluding node 'x'. In such scenario
> +		 * we should not promote to node 'x' from slow memory node.
> +		 */
>  		if (pol->flags & MPOL_F_MORON) {
> +			/*
> +			 * Optimize placement among multiple nodes
> +			 * via NUMA balancing
> +			 */
>  			if (node_isset(thisnid, pol->nodes))
>  				break;
>  			goto out;
>  		}
> -		fallthrough;
>  
> -	case MPOL_PREFERRED_MANY:
>  		/*
>  		 * use current page if in policy nodemask,
>  		 * else select nearest allowed node, if any.