Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Vitaly Wool]

On Wed, Jul 3, 2019 at 12:24 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 2 Jul 2019 15:17:47 -0700 Henry Burns <henryburns@xxxxxxxxxx> wrote:
>
> > > > > > +       if (can_sleep) {
> > > > > > +               lock_page(page);
> > > > > > +               __SetPageMovable(page, pool->inode->i_mapping);
> > > > > > +               unlock_page(page);
> > > > > > +       } else {
> > > > > > +               if (!WARN_ON(!trylock_page(page))) {
> > > > > > +                       __SetPageMovable(page, pool->inode->i_mapping);
> > > > > > +                       unlock_page(page);
> > > > > > +               } else {
> > > > > > +                       pr_err("Newly allocated z3fold page is locked\n");
> > > > > > +                       WARN_ON(1);
>
> The WARN_ON will have already warned in this case.
>
> But the whole idea of warning in this case may be undesirable.  We KNOW
> that the warning will sometimes trigger (yes?).  So what's the point in
> scaring users?

Well, normally a newly allocated page that we own should not be locked
by someone else so this is worth a warning IMO. With that said, the
else branch here appears to be redundant.

~Vitaly