On 3/22/21 6:59 AM, Michal Hocko wrote: > On Fri 19-03-21 15:42:02, Mike Kravetz wrote: >> The number of hugetlb pages can be adjusted by writing to the >> sysps/proc files nr_hugepages, nr_hugepages_mempolicy or >> nr_overcommit_hugepages. There is nothing to prevent two >> concurrent modifications via these files. The underlying routine >> set_max_huge_pages() makes assumptions that only one occurrence is >> running at a time. Specifically, alloc_pool_huge_page uses a >> hstate specific variable without any synchronization. > > From the above it is not really clear whether the unsynchronized nature > of set_max_huge_pages is really a problem or a mere annoynce. I suspect > the later because counters are properly synchronized with the > hugetlb_lock. It would be great to clarify that. > It is a problem and an annoyance. The problem is that alloc_pool_huge_page -> for_each_node_mask_to_alloc is called after dropping the hugetlb lock. for_each_node_mask_to_alloc uses the helper hstate_next_node_to_alloc which uses and modifies h->next_nid_to_alloc. Worst case would be two instances of set_max_huge_pages trying to allocate pages on different sets of nodes. Pages could get allocated on the wrong nodes. I really doubt this problem has ever been experienced in practice. However, when looking at the code in was a real annoyance. :) I'll update the commit message to be more clear. -- Mike Kravetz
