Re: [PATCH v4 04/13] mm/mempolicy: allow preferred code to take a nodemask

[Feng Tang]

On Wed, Apr 14, 2021 at 02:55:39PM +0200, Michal Hocko wrote:
> On Wed 17-03-21 11:40:01, Feng Tang wrote:
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > 
> > Create a helper function (mpol_new_preferred_many()) which is usable
> > both by the old, single-node MPOL_PREFERRED and the new
> > MPOL_PREFERRED_MANY.
> > 
> > Enforce the old single-node MPOL_PREFERRED behavior in the "new"
> > version of mpol_new_preferred() which calls mpol_new_preferred_many().
> > 
> > v3:
> >   * fix a stack overflow caused by emty nodemask (Feng)
> > 
> > Link: https://lore.kernel.org/r/20200630212517.308045-5-ben.widawsky@xxxxxxxxx
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > ---
> >  mm/mempolicy.c | 21 +++++++++++++++++++--
> >  1 file changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 1228d8e..6fb2cab 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -203,17 +203,34 @@ static int mpol_new_interleave(struct mempolicy *pol, const nodemask_t *nodes)
> >  	return 0;
> >  }
> >  
> > -static int mpol_new_preferred(struct mempolicy *pol, const nodemask_t *nodes)
> > +static int mpol_new_preferred_many(struct mempolicy *pol,
> > +				   const nodemask_t *nodes)
> >  {
> >  	if (!nodes)
> >  		pol->flags |= MPOL_F_LOCAL;	/* local allocation */
> 
> Now you have confused me. I thought that MPOL_PREFERRED_MANY for NULL
> nodemask will be disallowed as it is effectively MPOL_PREFERRED aka
> MPOL_F_LOCAL. Or do I misread the code?

I think you are right, with current code, the 'nodes' can't be NULL for
MPOL_PREFERRED_MANY, we'll revisit this.

And I have to admit that I am confused by the current logic for MPOL_PREFERRED,
that the nodemask paramter changes between raw user input, empty nodes and NULL.

Maybe the following patch can make it more clear, as it doesn't play the
NULL nmask trick?

---
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index be160d4..9cabfca 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -200,12 +200,9 @@ static int mpol_new_interleave(struct mempolicy *pol, const nodemask_t *nodes)
 
 static int mpol_new_preferred(struct mempolicy *pol, const nodemask_t *nodes)
 {
-	if (!nodes)
-		pol->flags |= MPOL_F_LOCAL;	/* local allocation */
-	else if (nodes_empty(*nodes))
+	if (nodes_empty(*nodes))
 		return -EINVAL;			/*  no allowed nodes */
-	else
-		pol->v.preferred_node = first_node(*nodes);
+	pol->v.preferred_node = first_node(*nodes);
 	return 0;
 }
 
@@ -239,9 +236,11 @@ static int mpol_set_nodemask(struct mempolicy *pol,
 		  cpuset_current_mems_allowed, node_states[N_MEMORY]);
 
 	VM_BUG_ON(!nodes);
-	if (pol->mode == MPOL_PREFERRED && nodes_empty(*nodes))
-		nodes = NULL;	/* explicit local allocation */
-	else {
+	if (pol->mode == MPOL_PREFERRED && nodes_empty(*nodes)) {
+		/* explicit local allocation */
+		pol->flags |= MPOL_F_LOCAL;
+		return 0;
+	} else {
 		if (pol->flags & MPOL_F_RELATIVE_NODES)
 			mpol_relative_nodemask(&nsc->mask2, nodes, &nsc->mask1);
 		else
@@ -254,10 +253,7 @@ static int mpol_set_nodemask(struct mempolicy *pol,
 						cpuset_current_mems_allowed;
 	}
 
-	if (nodes)
-		ret = mpol_ops[pol->mode].create(pol, &nsc->mask2);
-	else
-		ret = mpol_ops[pol->mode].create(pol, NULL);
+	ret = mpol_ops[pol->mode].create(pol, &nsc->mask2);
 	return ret;
 }


Thanks,
Feng