Michal Hocko <mhocko@xxxxxxxxxx> writes: > 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 > 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> > --- > > 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); > } > For both MPOL_PREFERED and MPOL_INTERLEAVE we pick the zone list from the node other than the current running node. Why don't we do that for MPOL_BIND ?ie, if the current node is not part of the policy node mask why are we not picking the first node from the policy node mask for MPOL_BIND ? -aneesh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>