Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <5fa8dfeb-8eb6-0de0-a335-e0b4336c29e0@I-love.SAKURA.ne.jp>
 <20230529190526.65d13658@kernel.org> <CANn89iLpeeJN=QA80c1wZZ0TB+ptzKN_Hq4Cf27C-URvKKbd2Q@mail.gmail.com>
 <c17ff3ff-b3af-fea2-f75e-68cefbc90636@I-love.SAKURA.ne.jp>
In-Reply-To: <c17ff3ff-b3af-fea2-f75e-68cefbc90636@I-love.SAKURA.ne.jp>
From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 30 May 2023 12:06:19 +0200
Message-ID: <CANn89iJbQ8rO8Mr-OaUcfXpcKiWEupuaT931NOsphQ-8BuPs5w@mail.gmail.com>
Subject: Re: [PATCH] ref_tracker: add stack_depot_save() failure handling to ref_tracker_alloc()
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Jakub Kicinski <kuba@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, May 30, 2023 at 11:52=E2=80=AFAM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> On 2023/05/30 16:22, Eric Dumazet wrote:
> >>> Therefore, assume that stack_depot_save(GFP_KERNEL | __GFP_NOFAIL) fr=
om
> >>> ref_tracker_alloc() can silently fail, and emit "unreliable refcount
> >>> tracker." message.
> >
> > Note: I never assumed stack_depot_save() would enforce/use NOFAIL.
>
> Hmm, I misread this function.
>
>         if (gfp & __GFP_DIRECT_RECLAIM)
>                 gfp_mask |=3D __GFP_NOFAIL; // Or'ing to "gfp_mask" than =
"gfp".
>         *trackerp =3D tracker =3D kzalloc(sizeof(*tracker), gfp_mask); //=
 <=3D This is "gfp_mask".
>         tracker->alloc_stack_handle =3D stack_depot_save(entries, nr_entr=
ies, gfp); // <=3D This is "gfp".
>
> So, stack_depot_save(GFP_KERNEL | __GFP_NOFAIL) is not happening.

Yes.1

>
> Then, question becomes whether we want tracker->alloc_stack_handle !=3D N=
ULL or not.
> If tracker->alloc_stack_handle =3D=3D NULL is still useful, this patch wi=
ll be useless...
>

I think it is useful to have the tracker (as Jakub hinted).
It is better than nothing.
We even might be able to allocate memory later for the
free_stack_handle which could give us
developers enough clues for bug hunting.

Thanks.