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

From: Feng Tang
Date: Mon Apr 19 2021 - 04:49:38 EST


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