On Fri, Jan 4, 2013 at 6:08 AM, Mel Gorman <mgorman@xxxxxxx> wrote: > Despite the reason for these commits, NUMA balancing is not the direct > source of the problem. split_huge_page() expected the anon_vma lock to be > exclusive to serialise the whole split operation. Ordinarily it is expected > that the anon_vma lock would only be required when updating the avcs but > THP also uses it. The locking requirements for THP are complex and there > is some overlap but broadly speaking they include the following > > 1. mmap_sem for read or write prevents THPs being created underneath > 2. anon_vma is taken for write if collapsing a huge page > 3. mm->page_table_lock should be taken when checking if pmd_trans_huge as > split_huge_page can run in parallel > 4. wait_split_huge_page uses anon_vma taken for write mode to serialise > against other THP operations > 5. compound_lock is used to serialise between > __split_huge_page_refcount() and gup > > split_huge_page takes anon_vma for read but that does not serialise against > parallel split_huge_page operations on the same page (rule 2). One process > could be modifying the ref counts while the other modifies the page tables > leading to counters not being reliable. This patch takes the anon_vma > lock for write to serialise against parallel split_huge_page and parallel > collapse operations as it is the most fine-grained lock available that > protects against both. Your comment about this being the most fine-grained lock made me think, couldn't we use lock_page() on the THP page here ? Now I don't necessarily want to push you that direction, because I haven't fully thought it trough and because what you propose brings us closer to what happened before anon_vma became an rwlock, which is more obviously safe. But I felt I should still mention it, since we're really only trying to protect from concurrent operations on the same THP page, so locking at just that granularity would seem desirable. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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>