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

[Henry Burns]

On Tue, Jul 2, 2019 at 10:54 PM Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
>
> 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.
The else branch has been removed, and I think it's possible (albeit unlikely)
that the trylock could fail due to either compaction or kstaled
(In which case the page just won't be movable).

Also Vitaly, do you have a preference between the two emails? I'm not sure which
one to include.