Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim

[Yu Zhao <yuzhao@google.com>]

On Thu, Jan 4, 2024 at 1:48 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 03-01-24 18:07:43, Yu Zhao wrote:
> > On Wed, Jan 03, 2024 at 01:19:59PM -0500, Dan Schatzberg wrote:
> > > On Wed, Jan 03, 2024 at 10:19:40AM -0700, Yu Zhao wrote:
> > > [...]
> > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > index d91963e2d47f..394e0dd46b2e 100644
> > > > > --- a/mm/vmscan.c
> > > > > +++ b/mm/vmscan.c
> > > > > @@ -92,6 +92,11 @@ struct scan_control {
> > > > >         unsigned long   anon_cost;
> > > > >         unsigned long   file_cost;
> > > > >
> > > > > +#ifdef CONFIG_MEMCG
> > > > > +       /* Swappiness value for proactive reclaim. Always use sc_swappiness()! */
> > > > > +       int *proactive_swappiness;
> > > > > +#endif
> > > >
> > > > Why is proactive_swappiness still a pointer? The whole point of the
> > > > previous conversation is that sc->proactive can tell whether
> > > > sc->swappiness is valid or not, and that's less awkward than using a
> > > > pointer.
> > >
> > > It's the same reason as before - zero initialization ensures that the
> > > pointer is NULL which tells us if it's valid or not. Proactive reclaim
> > > might not set swappiness and you need to distinguish swappiness of 0
> > > and not-set. See this discussion with Michal:
> > >
> > > https://lore.kernel.org/linux-mm/ZZUizpTWOt3gNeqR@tiehlicka/
> >
> >  static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> >                               size_t nbytes, loff_t off)
> >  {
> >         struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> >         unsigned int nr_retries = MAX_RECLAIM_RETRIES;
> >         unsigned long nr_to_reclaim, nr_reclaimed = 0;
> > +       int swappiness = -1;
> > ...
> >                 reclaimed = try_to_free_mem_cgroup_pages(memcg,
> >                                         min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> > -                                       GFP_KERNEL, reclaim_options);
> > +                                       GFP_KERNEL, reclaim_options,
> > +                                       swappiness);
> >
> > ...
> >
> > +static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> > +{
> > +       return sc->proactive && sc->proactive_swappiness > -1 ?
> > +              sc->proactive_swappiness : mem_cgroup_swappiness(memcg);
> > +}
>
> Tpo be completely honest I really fail to see why this is such a hot
> discussion point. To be completely clear both approaches are feasible.

Feasible but not equal.

> The main argument for NULL check based approach is that it is less error
> prone from an incorrect ussage because any bug becomes obvious.

Any bug becomes *fatal*, and fatal isn't only obvious but also hurts
in production systems.

This was the reason for going through the trouble switching from
VM_BUG_ON() to VM_WARN_ON() and documenting it in
Documentation/process/coding-style.rst:

22) Do not crash the kernel
---------------------------

In general, the decision to crash the kernel belongs to the user, rather
than to the kernel developer.

Isn't?

> If we
> use any other special constant a missing initialization would be much
> harder to spot because they would be subtle behavior change.
>
> Are there really any strong arguments to go against this "default
> initialization is safe" policy?

Just wanted to point out an alternative. Fine details (best practices)
matter to me.