On Sat, Jan 05, 2013 at 06:24:47AM -0600, Simon Jeons wrote: > On Fri, 2013-01-04 at 17:32 -0800, Michel Lespinasse wrote: > > 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. > > Why you said that anon_vma lock who will protect page associated to a > list of vmas is fine-grained then page lock who just protect one page? > We did not say it was fine-grained, we said it was the most fine-grained lock available. It's a coarse lock but using the page lock would be problematic. -- Mel Gorman SUSE Labs -- 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>