Re: [PATCH v3 3/5] mm: madvise: implement lightweight guard page mechanism

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Oct 25, 2024 at 11:35:03PM +0100, Lorenzo Stoakes wrote:
> On Fri, Oct 25, 2024 at 11:56:52PM +0200, Vlastimil Babka wrote:
> > On 10/25/24 19:12, Lorenzo Stoakes wrote:
> > > On Wed, Oct 23, 2024 at 05:24:40PM +0100, Lorenzo Stoakes wrote:
> > >> Implement a new lightweight guard page feature, that is regions of userland
> > >> virtual memory that, when accessed, cause a fatal signal to arise.
> > >
> > > <snip>
> > >
> > > Hi Andrew - Could you apply the below fix-patch? I realise we must handle
> > > fatal signals and conditional rescheduling in the vector_madvise() special
> > > case.
> > >
> > > Thanks!
> > >
> > > ----8<----
> > > From 546d7e1831c71599fc733d589e0d75f52e84826d Mon Sep 17 00:00:00 2001
> > > From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > Date: Fri, 25 Oct 2024 18:05:48 +0100
> > > Subject: [PATCH] mm: yield on fatal signal/cond_sched() in vector_madvise()
> > >
> > > While we have to treat -ERESTARTNOINTR specially here as we are looping
> > > through a vector of operations and can't simply restart the entire
> > > operation, we mustn't hold up fatal signals or RT kernels.
> >
> > For plain madvise() syscall returning -ERESTARTNOINTR does the right thing
> > and checks fatal_signal_pending() before returning, right?
>
> I believe so. But now you've caused me some doubt so let me double check
> and make absolutely sure :)
>
> >
> > Uh actually can we be just returning -ERESTARTNOINTR or do we need to use
> > restart_syscall()?
>
> Yeah I was wondering about that, but restart_syscall() seems to set
> TIF_SIGPENDING, and I wondered if that was correct... but then I saw other
> places that seemed to use it direct so it seemed so.
>
> Let's eliminiate doubt, will check this next week and make sure.
>

Yeah looks like we do actually, as the function is handled by
arch_do_signal_or_restart():

do_syscall_64()
  -> sycall_exit_to_user_mode_work()
    -> __sycall_exit_to_user_mode_work()
      -> exit_to_user_mode_prepare()
        -> exit_to_user_mode_loop()
	  -> arch_do_signal_or_restart()
	    -> (possibly) get_signal()

And arch_do_signal_or_restart() is only invoked by exit_to_user_mode_loop()
if _TIF_SIGPENDING or _TIF_NOTIFY_SIGNAL is set:

		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
			arch_do_signal_or_restart(regs);

This is set by restart_syscall():

static inline int restart_syscall(void)
{
	set_tsk_thread_flag(current, TIF_SIGPENDING);
	return -ERESTARTNOINTR;
}

It's a nop if no signal is actually pending too, and it handily also deals
with signals...

> >
> > > ---
> > >  mm/madvise.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 48eba25e25fe..127aa5d86656 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -1713,8 +1713,14 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> > >  		 * we have already rescinded locks, it should be no problem to
> > >  		 * simply try again.
> > >  		 */
> > > -		if (ret == -ERESTARTNOINTR)
> > > +		if (ret == -ERESTARTNOINTR) {
> > > +			if (fatal_signal_pending(current)) {
> > > +				ret = -EINTR;
> > > +				break;
> > > +			}
> > > +			cond_resched();
> >
> > Should be unnecessary as we're calling an operation that takes a rwsem so
> > there are reschedule points already. And with lazy preempt hopefully
> > cond_resched()s will become history, so let's not add more only to delete later.
>
> Ack will remove on respin.
>
> >
> > >  			continue;
> > > +		}
> > >  		if (ret < 0)
> > >  			break;
> > >  		iov_iter_advance(iter, iter_iov_len(iter));
> > > --
> > > 2.47.0
> >
>
> For simplicitly with your other comments too I think I'll respin this next
> week.

So will respin to use restart_syscall() correctly (+ fix up your other points too).

Have tested and confirmed that failing to use restart_syscall() causes the
-ERESTARTINTR to be returned and no restart, but using it works.