Re: swapaccount= commandline option

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Feb 13, 2024 at 12:09:26AM -0800, Yosry Ahmed wrote:
> On Tue, Feb 13, 2024 at 12:06 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Tue 13-02-24 08:01:37, Narcis Garcia wrote:
> > > El 12/2/24 a les 23:37, Yosry Ahmed ha escrit:
> > > > On Sun, Feb 11, 2024 at 1:33 AM Narcis Garcia <debianlists@actiu.net> wrote:
> > > > >
> > > > > Hello,
> > > > >
> > > > > I read this warning on Linux 6.1 boot:
> > > > > The swapaccount= commandline option is deprecated. Please report your
> > > > > usecase to linux-mm@kvack.org if you depend on this functionality.
> > > > >
> > > > > And I want to know if this impacts on LinuX Containers (LXC) swap
> > > > > accounting.
> > > >
> > > > swapaccount= was deprecated in the sense that it is always on now
> > > > (i.e. you cannot do swapaccount=0 anymore). I believe you are using
> > > > swapaccount=1 and want to make sure that swap accounting will still
> > > > work with v6.1, right?
> > > >
> > > > This is the second question we have had about this message [1]. I
> > > > think we need to clarify the warning (and perhaps backport to stable
> > > > kernels).
> > > >
> > > > [1]https://lore.kernel.org/linux-mm/2593755.Xrv9ptJ69j@sinistra.local/
> > >
> > >
> > > Thank you for answering this question, and sure this clarification is needed
> > > both in documentation and warning message.
> > > I've found deprecation message was commited on 26 Sep 2022.
> > > From what Linux version release, this deprecation is included?
> > >
> > > I suggest this for setup_swap_account() code:
> > > pr_warn_once("The swapaccount= commandline option is deprecated. "
> > >       "Swap accounting is always on. "
> > >       "Please report your usecase to linux-mm@kvack.org if you "
> > >       "depend on this functionality.\n");
> >
> > Would it make more sense to warn only if somebody tries to disable the
> > swap accounting?
> > ---
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e4c8735e7c85..dec75eedf634 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -7936,7 +7936,10 @@ bool mem_cgroup_swap_full(struct folio *folio)
> >
> >  static int __init setup_swap_account(char *s)
> >  {
> > -       pr_warn_once("The swapaccount= commandline option is deprecated. "
> > +       bool res;
> > +
> > +       if (kstrtobool(s, &res) || !res)
> > +               pr_warn_once("The swapaccount= commandline option is deprecated. "
> >                      "Please report your usecase to linux-mm@kvack.org if you "
> >                      "depend on this functionality.\n");
> >         return 1;
> > --
> 
> Yes, this makes sense to me, but I still think explicitly specifying
> that swap accounting is always on now would make things even clearer.

I think it's okay if it only warns when somebody requests disabling,
since the confusion won't happen anymore for folks still explicitly
requesting the enablement. I did add a blurb about what the preferred
method to control swap behavior is, though.