linux-kernel - Re: [PATCH] mm: memcomtrol: add warning in case of rtpn = NULL

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Message-ID: <ZONz8Zqwg4cuCGCe@casper.infradead.org>
Date:   Mon, 21 Aug 2023 15:25:53 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Anastasia Belova <abelova@...ralinux.ru>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH] mm: memcomtrol: add warning in case of rtpn = NULL

On Mon, Aug 21, 2023 at 05:20:51PM +0300, Anastasia Belova wrote:
> kzalloc_node may return NULL. So rtpn should be
> checked before dereference in mem_cgroup_init.

What good will this do?

>  
>  		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL, node);
> +		WARN_ON(!rtpn);
>  
>  		rtpn->rb_root = RB_ROOT;

so you'll get a warning right before you get a crash, which will provide
exactly the same information.  And you'd already have a warning that
the memory allocation had failed (because __GFP_NOWARN wasn't set).
So you've increased the amount of data from 2 dumps to 3 without providing
any neww information.

If you _did_ something to avoid the crash, there might be a point to
this, but the fact is that small GFP_KERNEL allocations _don't_ fail.
You need to teach your tool that, or you'll be overwhelmed with
false positives.