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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH v6 00/19] The new cgroup slab memory controller
Date:   Sun, 21 Jun 2020 19:53:23 -0400
Message-Id: <7DEA68BC-3B9A-46FA-8103-AE6B72324591@lca.pw>
References: <20200621233429.GA293939@carbon.DHCP.thefacebook.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        Vlastimil Babka <vbabka@suse.cz>, kernel-team@fb.com,
        linux-kernel@vger.kernel.org, catalin.marinas@arm.com
In-Reply-To: <20200621233429.GA293939@carbon.DHCP.thefacebook.com>
To:     Roman Gushchin <guro@fb.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jun 21, 2020, at 7:34 PM, Roman Gushchin <guro@fb.com> wrote:
>=20
> My wild guess is that kmemleak is getting confused by modifying the lowest=

> bit of page->mem_cgroup/obhj_cgroups pointer:
>=20
> struct page {
>    ...
>    union {
>        struct mem_cgroup *mem_cgroup;
>        struct obj_cgroup **obj_cgroups;
>    };
>    ...
> }
>=20
> We're using the lowest bit to distinguish between a "normal" mem_cgroup
> pointer and a vector of obj_cgroup pointers.
>=20
> This pointer to obj_cgroup vector is saved only here, so if we're modifyin=
g
> the address, I guess it's what makes kmemleak think that there is a leak.
>=20
> Or do you have a real leak?

The point is that we can=E2=80=99t have a patchset in the current form to to=
tally render kmemleak useless with so many even false positives.

Anyway, this is rather easy to reproduce where I am able to reproduce on mul=
tiple bare-metal machines by just booting it.

# echo scan > /sys/kernel/debug/kmemleak
# cat /sys/kernel/debug/kmemleak=