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From:   Vitaly Wool <vitalywool@gmail.com>
Date:   Wed, 3 Jul 2019 07:53:32 +0200
Message-ID: <CAMJBoFOhXP36L6pZEA-7p24mJweDGe9iYb2fo1nNCxadYHcPzQ@mail.gmail.com>
Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Henry Burns <henryburns@google.com>,
        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 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.

~Vitaly