On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote:
> On Wed 26-05-21 13:01:42, Feng Tang wrote:
> > Now the only remaining case of a real 'local' policy faked by
> > 'prefer' policy plus MPOL_F_LOCAL bit is:
> >
> > A valid 'prefer' policy with a valid 'preferred' node is 'rebind'
> > to a nodemask which doesn't contains the 'preferred' node, then it
> > will handle allocation with 'local' policy.
> >
> > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the
> > MPOL_F_LOCAL bit, which could simplify the code much.
>
> As I've pointed out in the reply to the previous patch. It would have
> been much better if most of the MPOL_F_LOCAL usage was gone by this
> patch.
>
> I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the
> hack back in after you have painstakingly removed it. So this looks like
> a step backwards to me. I also do not understand why do we need the
> rebind callback for local policy at all. There is no node mask for local
> so what is going on here?
This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES
flag set, say it prefer node 1, when it is later 'refind' to a new
nodemask node 2-3, according to current code it will be add the
MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future
it is 'rebind' again with a nodemask 1-2, it will be restored back
to 'prefer' policy with preferred node 1.
This patch tries to address this special case. I have struggled but
couldn't think of a good way. Any suggestions? thanks!
- Feng
> [...]
> > +static void mpol_rebind_local(struct mempolicy *pol,
> > + const nodemask_t *nodes)
> > +{
> > + if (unlikely(pol->flags & MPOL_F_STATIC_NODES)) {
> > + int node = first_node(pol->w.user_nodemask);
> > +
> > + BUG_ON(!(pol->flags & MPOL_F_LOCAL_TEMP));
> > +
> > + if (node_isset(node, *nodes)) {
> > + pol->v.preferred_node = node;
> > + pol->mode = MPOL_PREFERRED;
> > + pol->flags &= ~MPOL_F_LOCAL_TEMP;
> > + }
> > + }
> > +}
> > +
>
> I have to confess I've got lost here. Could you explain why do you need
> all this handling for a local policy?
>
> > static void mpol_rebind_preferred(struct mempolicy *pol,
> > const nodemask_t *nodes)
> > {
> > @@ -347,13 +363,19 @@ static void mpol_rebind_preferred(struct mempolicy *pol,
> >
> > if (node_isset(node, *nodes)) {
> > pol->v.preferred_node = node;
> > - pol->flags &= ~MPOL_F_LOCAL;
> > - } else
> > - pol->flags |= MPOL_F_LOCAL;
> > + } else {
> > + /*
> > + * If there is no valid node, change the mode to
> > + * MPOL_LOCAL, which will be restored back when
> > + * next rebind() sees a valid node.
> > + */
> > + pol->mode = MPOL_LOCAL;
> > + pol->flags |= MPOL_F_LOCAL_TEMP;
> > + }
> > } else if (pol->flags & MPOL_F_RELATIVE_NODES) {
> > mpol_relative_nodemask(&tmp, &pol->w.user_nodemask, nodes);
> > pol->v.preferred_node = first_node(tmp);
> > - } else if (!(pol->flags & MPOL_F_LOCAL)) {
> > + } else {
> > pol->v.preferred_node = node_remap(pol->v.preferred_node,
> > pol->w.cpuset_mems_allowed,
> > *nodes);
> > @@ -372,7 +394,7 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask)
> > {
> > if (!pol)
> > return;
> > - if (!mpol_store_user_nodemask(pol) && !(pol->flags & MPOL_F_LOCAL) &&
> > + if (!mpol_store_user_nodemask(pol) &&
> > nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
> > return;
> >
> > @@ -425,7 +447,7 @@ static const struct mempolicy_operations mpol_ops[MPOL_MAX] = {
> > },
> > [MPOL_LOCAL] = {
> > .create = mpol_new_local,
> > - .rebind = mpol_rebind_default,
> > + .rebind = mpol_rebind_local,
> > },
> > };
> --
> Michal Hocko
> SUSE Labs
