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From:   Henry Burns <henryburns@google.com>
Date:   Wed, 3 Jul 2019 09:39:12 -0700
Message-ID: <CAGQXPTgRC23SHoKZTctkJsEJORu7GHDYNz_+9HaDu9ntffrzig@mail.gmail.com>
Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
To:     Vitaly Wool <vitalywool@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vitaly Vul <vitaly.vul@sony.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Xidong Wang <wangxidong_97@163.com>,
        Jonathan Adams <jwadams@google.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jul 2, 2019 at 10:54 PM Vitaly Wool <vitalywool@gmail.com> wrote:
>
> On Wed, Jul 3, 2019 at 12:24 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Tue, 2 Jul 2019 15:17:47 -0700 Henry Burns <henryburns@google.com> 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.