Re: [PATCH 1/2] mm: fix the racy mm->locked_vm change in

[Hugh Dickins]

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 <hughd@xxxxxxxxxx>
> 
> 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
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