On Fri, Apr 26, 2024 at 05:58:16PM -0700, Yosry Ahmed wrote:
> On Fri, Apr 26, 2024 at 5:38 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> >
> > To reduce memory usage by the memcg events and stats, the kernel uses
> > indirection table and only allocate stats and events which are being
> > used by the memcg code. To make this more robust, let's add warnings
> > where unexpected stats and events indexes are used.
> >
> > Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > ---
> > mm/memcontrol.c | 43 ++++++++++++++++++++++++++++++++++---------
> > 1 file changed, 34 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 103e0e53e20a..36145089dcf5 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -671,9 +671,11 @@ unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
> > return node_page_state(lruvec_pgdat(lruvec), idx);
> >
> > i = memcg_stats_index(idx);
> > - if (i >= 0) {
> > + if (likely(i >= 0)) {
> > pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> > x = READ_ONCE(pn->lruvec_stats->state[i]);
> > + } else {
> > + pr_warn_once("%s: stat item index: %d\n", __func__, idx);
> > }
>
> Can we make these more compact by using WARN_ON_ONCE() instead:
>
> if (WARN_ON_ONCE(i < 0))
> return 0;
>
> I guess the advantage of using pr_warn_once() is that we get to print
> the exact stat index, but the stack trace from WARN_ON_ONCE() should
> make it obvious in most cases AFAICT.
>
> No strong opinions either way.
One reason I used pr_warn_once() over WARN_ON_ONCE() is the syzbot
trigger. No need to trip the bot over this error condition.
