On Sun 24-11-24 03:09:35, Junjie Fu wrote: > When handling a page fault caused by NUMA balancing (do_numa_page), it is > necessary to decide whether to migrate the current page to another node or > keep it on its current node. For pages with the MPOL_PREFERRED memory > policy, it is sufficient to check whether the first node set in the > nodemask is the same as the node where the page is currently located. If > this is the case, the page should remain in its current state. Otherwise, > migration to another node should be attempted. > > Because the definition of MPOL_PREFERRED is as follows: "This mode sets the > preferred node for allocation. The kernel will try to allocate pages from > this node first and fall back to nearby nodes if the preferred node is low > on free memory. If the nodemask specifies more than one node ID, the first > node in the mask will be selected as the preferred node." > > Thus, if the node where the current page resides is not the first node in > the nodemask, it is not the PREFERRED node, and memory migration can be > attempted. > > However, in the original code, the check only verifies whether the current > node exists in the nodemask (which may or may not be the first node in the > mask). This could lead to a scenario where, if the current node is not the > first node in the nodemask, the code incorrectly decides not to attempt > migration to other nodes. > > This behavior is clearly incorrect. If the target node for migration and > the page's current NUMA node are both within the nodemask but neither is > the first node, they should be treated with the same priority, and > migration attempts should proceed. The code is clearly confusing but is there any actual problem to be solved? IIRC although we do keep nodemask for MPOL_PREFERRED policy we do not allow to set more than a single node to be set there. Have a look at mpol_new_preferred -- Michal Hocko SUSE Labs
