On Thu, 1 Oct 2015, Oleg Nesterov wrote: > On 09/30, Hugh Dickins wrote: > > > > On Tue, 29 Sep 2015, Oleg Nesterov wrote: > > > > > "mm->locked_vm += grow" and vm_stat_account() in acct_stack_growth() > > > are not safe; multiple threads using the same ->mm can do this at the > > > same time trying to expans different vma's under down_read(mmap_sem). > > expand > > > This means that one of the "locked_vm += grow" changes can be lost > > > and we can miss munlock_vma_pages_all() later. > > > > From the Cc list, I guess you are thinking this might be the fix to > > the "Bad state page (mlocked)" issues Andrey and Sasha have reported. > > Yes, I found this when I tried to explain this problem, but I doubt > this change can fix it... Firstly I think it is very unlikely that > trinity hits this race. And even if mm->locked_vm is wrongly equal > to zero in exit_mmap(), it seems that page_remove_rmap() should do > clear_page_mlock().
Oh yes, good point, a subsequent clear_page_mlock(), in unmapping this address space, or later unmapping from another, ought to clear it before the page ever gets freed. > But I do not understand this code enough. So if > this patch can actually help I would really like to know why ;) I doubt any of us understand it very well, mlock+munlock have over the years become so much more grotesque than the uninitiated would expect. > > And of course this can not explain other traces which look like > mm->mmap corruption. > > > Acked-by: Hugh Dickins <hu...@google.com> > > Thanks! > > > with some hesitation. I don't like very much that the preliminary > > mm->locked_vm + grow check is still done without complete locking, > > so racing threads could get more locked_vm than they're permitted; > > but I'm not sure that we care enough to put page_table_lock back > > over all of that (and security_vm_enough_memory wants to have final > > say on whether to go ahead); even if it was that way years ago. > > Yes. Plus all these RLIMIT_MEMLOCK/etc and security_* checks assume > that we are going to expand current->mm, but this is not necessarily > true. Debugger or sys_process_vm_* can expand a foreign vma. Right, I'd forgotten all about that aspect: yes, none of us ever took expand_stack()'s "current" assumptions seriously enough to rework its interface with all the architectures, so that's another argument for sticking for now with the patch you already have here - thanks. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
