On Wed 26-05-21 13:01:39, Feng Tang wrote: > mempolicy_nodemask_intersects() is used in oom case to check if a > task may have memory allocated on some memory nodes. > > Currently, the nodes_intersects() is run for both 'bind' and 'interleave' > policies. But they are different regarding memory allocation, the nodemask > is a forced requirement for 'bind', while just a hint for 'interleave'. > Like in alloc_pages_vma(): > > nmask = policy_nodemask(gfp, pol); > preferred_nid = policy_node(gfp, pol, node); > page = __alloc_pages(gfp, order, preferred_nid, nmask); > > in plicy_nodemask(), only 'bind' policy may return its desired nodemask, > while others return NULL. And this 'NULL' enables the 'interleave' policy > can get memory from other nodes than its nodemask. > > So skip the nodemask intersect check for 'interleave' policy. The changelog is not really clear on the actual effect of the patch and the above reference to alloc_pages_vma looks misleading to me because that path is never called for interleaved policy. This is very likely my fault because I was rather vague. The existing code in its current form is confusing but it _works_ properly. The problem is that it sounds like a general helper and in that regards the function is correct for the interleaved policy and your proposed preferred-many. But its only existing caller wants a different semantic. Until now this was not a real problem even for OOM context because alloc_page_interleave is always used for the interleaving policy and that one doesn't use any node mask so the code is not really exercised. With your MPOL_PREFERRED this would no longer be the case. Your patch makes the code more robust for the oom context but it can confuse other users who really want to do an intersect logic. So I think it would really be best to rename the function and make it oom specific. E.g. mempolicy_in_oom_domain(tsk, mask) this would make it clear that this is not a general purpose function. The changelog should be clear that this is just a code cleanup rather than fix. -- Michal Hocko SUSE Labs
