Re: [PATCH] mm/mmzone.c: fix page_cpupid_xchg_last() to READ_ONCE() the page flags

[Peter Collingbourne <pcc@google.com>]

On Wed, Jan 19, 2022 at 2:03 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jan 18, 2022 at 03:05:39PM -0800, Peter Collingbourne wrote:
> > After submitting a patch with a compare-exchange loop similar to this
> > one to set the KASAN tag in the page flags, Andrey Konovalov pointed
> > out that we should be using READ_ONCE() to read the page flags. Fix
> > it here.
>
> What does it actually fix? If it manages to split the read and read
> garbage the cmpxchg will fail and we go another round, no harm done.

What I wasn't sure about was whether the compiler would be allowed to
break this code by hoisting the read of page->flags out of the loop
(because nothing in the loop actually writes to page->flags aside from
the compare-exchange, and if that succeeds we're *leaving* the loop).
That could potentially result in a loop that never terminates if the
first compare-exchange fails. This is largely a theoretical problem as
far as I know; the assembly produced by clang and gcc on x86_64 and
arm64 appears to be doing the expected thing for now, and we're using
inline asm for compare-exchange instead of the compiler builtins on
those architectures (and on all other architectures it seems? no
matches for __atomic_compare_exchange outside of kcsan and the
selftests) so the compiler wouldn't be able to look inside it anyway.

> > Fixes: 75980e97dacc ("mm: fold page->_last_nid into page->flags where possible")
>
> As per the above argument, I don't think this rates a Fixes tag, there
> is no actual fix.

Okay, I'll remove it unless you find the above convincing.

> > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693
>
> That's that doing here?

I upload my changes to Gerrit and link to them here so that I (and
others) can see the progression of the patch via the web UI.

> > Cc: stable@vger.kernel.org
>
> That's massively over-selling things.

Fair enough since it isn't causing an actual problem, I'll remove this tag.

> > ---
> >  mm/mmzone.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/mmzone.c b/mm/mmzone.c
> > index eb89d6e018e2..f84b84b0d3fc 100644
> > --- a/mm/mmzone.c
> > +++ b/mm/mmzone.c
> > @@ -90,7 +90,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
> >       int last_cpupid;
> >
> >       do {
> > -             old_flags = flags = page->flags;
> > +             old_flags = flags = READ_ONCE(page->flags);
> >               last_cpupid = page_cpupid_last(page);
> >
> >               flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
>
> I think that if you want to touch that code, something like the below
> makes more sense...

Yeah, that looks a bit nicer. I'll send a v2 and update the other patch as well.

Peter