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

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

On Fri, Oct 25, 2024 at 11:56:52PM +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.

>
> > ---
> >  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  		 * we have already rescinded locks, it should be no problem to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.