linux-kernel - Re: [PATCH v6 00/19] The new cgroup slab memory controller

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Message-ID: <20200622030729.GB293939@carbon.DHCP.thefacebook.com>
Date:   Sun, 21 Jun 2020 20:07:29 -0700
From:   Roman Gushchin <guro@...com>
To:     Qian Cai <cai@....pw>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, <catalin.marinas@....com>
Subject: Re: [PATCH v6 00/19] The new cgroup slab memory controller

On Sun, Jun 21, 2020 at 07:53:23PM -0400, Qian Cai wrote:
> 
> 
> > On Jun 21, 2020, at 7:34 PM, Roman Gushchin <guro@...com> wrote:
> > 
> > My wild guess is that kmemleak is getting confused by modifying the lowest
> > bit of page->mem_cgroup/obhj_cgroups pointer:
> > 
> > struct page {
> >    ...
> >    union {
> >        struct mem_cgroup *mem_cgroup;
> >        struct obj_cgroup **obj_cgroups;
> >    };
> >    ...
> > }
> > 
> > We're using the lowest bit to distinguish between a "normal" mem_cgroup
> > pointer and a vector of obj_cgroup pointers.
> > 
> > This pointer to obj_cgroup vector is saved only here, so if we're modifying
> > the address, I guess it's what makes kmemleak think that there is a leak.
> > 
> > Or do you have a real leak?
> 
> The point is that we can’t have a patchset in the current form to totally render kmemleak useless with so many even false positives.
> 
> Anyway, this is rather easy to reproduce where I am able to reproduce on multiple bare-metal machines by just booting it.
> 
> # echo scan > /sys/kernel/debug/kmemleak
> # cat /sys/kernel/debug/kmemleak

Ok, thank you for the report, I'll take care of it.

It's easy to mark these vectors to be ignored by kmemleak, but I guess it's better
to explicitly add an additional reference, so we can track actual leaks.

I'll send a patch with fix soon-ish.

Thanks!