linux-kernel - Re: [PATCH] cgroup/misc: Fix an overflow

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Message-ID: <ZLWmdBfcuPUBtk1K@slm.duckdns.org>
Date:   Mon, 17 Jul 2023 10:37:08 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Haitao Huang <haitao.huang@...ux.intel.com>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>, dave.hansen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        cgroups@...r.kernel.org, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, vipinsh@...gle.com,
        kai.huang@...el.com, reinette.chatre@...el.com,
        zhiquan1.li@...el.com, kristen@...ux.intel.com
Subject: Re: [PATCH] cgroup/misc: Fix an overflow

Hello,

On Mon, Jul 17, 2023 at 03:19:38PM -0500, Haitao Huang wrote:
> Actually, we are using atomic_long_t for 'current' which is the same width
> as long defined by arch/compiler. So new_usage should be long to be
> consistent?

We can use atomic64_t, right? It's slower on 32bit machines but I think it'd
be better to guarantee resource counter range than micro-optimizing charge
operations. None of the current users are hot enough for this to matter and
if somebody becomes that hot, the difference between atomic_t and atomic64_t
isn't gonna matter that much. We'd need to batch allocations per-cpu and so
on.

> ditto for event counter. Only max is plain unsigned long but I think it is
> also OK as it only compared with 'current' without any arithmetic ops
> involved.
> Did I miss something here?

I'm saying that it'd be better to make everything explicitly 64bit.

Thanks.

-- 
tejun