Re: [PATCH] mm, mempolicy: clean up __GFP_THISNODE confusion in policy_zonelist

[Vlastimil Babka]

On 10/13/2016 02:59 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> __GFP_THISNODE is documented to enforce the allocation to be satisified
> from the requested node with no fallbacks or placement policy
> enforcements. policy_zonelist seemingly breaks this semantic if the
> current policy is MPOL_MBIND and instead of taking the node it will
> fallback to the first node in the mask if the requested one is not in
> the mask. This is confusing to say the least because it fact we
> shouldn't ever go that path. First tasks shouldn't be scheduled on CPUs
> with nodes outside of their mempolicy binding. And secondly
> policy_zonelist is called only from 3 places:
> - huge_zonelist - never should do __GFP_THISNODE when going this path
> - alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either
> - alloc_pages_current - which uses default_policy id __GFP_THISNODE is

						    ^ if

>   used
>
> So we shouldn't even need to care about this possibility and can drop
> the confusing code. Let's keep a WARN_ON_ONCE in place to catch
> potential users and fix them up properly (aka use a different allocation
> function which ignores mempolicy).
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Looks good, and a BUG_ON() removed as a bonus :)
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>
> Hi,
> I have noticed this while discussing this code [1]. The code as is
> quite confusing and I think it is worth cleaning up. I decided to be
> conservative and keep at least WARN_ON_ONCE if we have some caller which
> relies on __GFP_THISNODE in a mempolicy context so that we can fix it up.
>
> [1] http://lkml.kernel.org/r/57FE0184.6030008@xxxxxxxxxxxxxxxxxx
>
>  mm/mempolicy.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index ad1c96ac313c..33a305397bd4 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1679,25 +1679,17 @@ static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
>  static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy,
>  	int nd)
>  {
> -	switch (policy->mode) {
> -	case MPOL_PREFERRED:
> -		if (!(policy->flags & MPOL_F_LOCAL))
> -			nd = policy->v.preferred_node;
> -		break;
> -	case MPOL_BIND:
> +	if (policy->mode == MPOL_PREFERRED && !(policy->flags & MPOL_F_LOCAL))
> +		nd = policy->v.preferred_node;
> +	else {
>  		/*
> -		 * Normally, MPOL_BIND allocations are node-local within the
> -		 * allowed nodemask.  However, if __GFP_THISNODE is set and the
> -		 * current node isn't part of the mask, we use the zonelist for
> -		 * the first node in the mask instead.
> +		 * __GFP_THISNODE shouldn't even be used with the bind policy because
> +		 * we might easily break the expectation to stay on the requested node
> +		 * and not break the policy.
>  		 */
> -		if (unlikely(gfp & __GFP_THISNODE) &&
> -				unlikely(!node_isset(nd, policy->v.nodes)))
> -			nd = first_node(policy->v.nodes);
> -		break;
> -	default:
> -		BUG();
> +		WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
>  	}
> +
>  	return node_zonelist(nd, gfp);
>  }