On Wed, Aug 30, 2017 at 07:19:30AM +1000, Benjamin Herrenschmidt wrote: > On Tue, 2017-08-29 at 13:27 +0200, Peter Zijlstra wrote: > > mpe helped me out and explained that is the PWC hint to TBLIE. > > > > So, you set need_flush_all when you unhook pud/pmd/pte which you then > > use to set PWC. So free_pgtables() will do the PWC when it unhooks > > higher level pages. > > > > But you're right that there's some issues, free_pgtables() itself > > doesn't seem to use mm->page_table_lock,pmd->lock _AT_ALL_ to unhook the > > pages. > > > > If it were to do that, things should work fine since those locks would > > then serialize against the speculative faults, we would never install a > > page if the VMA would be under tear-down and it would thus not be > > visible to your caches either. > > That's one case. I don't remember of *all* the cases to be honest, but > I do remember several times over the past few years thinking "ah we are > fine because the mm sem taken for writing protects us from any > concurrent tree structure change" :-) Well, installing always seems to use the locks (it needs to, because its always done with down_read()), that only leaves removal, and the only place I know that removes stuff is free_pgtables(). But I think I found another fun place, copy_page_range(). While it (pointlessly) takes all the PTLs on the dst mm it walks the src page tables without any PTLs. This means that if we have a multi-threaded process doing fork() a thread of the src mm could instantiate page-tables that will not be copied over. Of course, this is highly dubious behaviour to begin with, and I don't think there's anything fundamentally wrong with missing those pages but we should document this stuff. -- 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>
