linux-kernel - Re: [PATCH 4/5] cgroup: Removing racy check in test_memcg

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Message-ID: <YmM/NCX9FwUY/GvB@carbon>
Date:   Fri, 22 Apr 2022 16:50:12 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     David Vernet <void@...ifault.com>
Cc:     akpm@...ux-foundation.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, hannes@...xchg.org, mhocko@...nel.org,
        shakeelb@...gle.com, kernel-team@...com
Subject: Re: [PATCH 4/5] cgroup: Removing racy check in test_memcg_sock()

On Fri, Apr 22, 2022 at 08:57:28AM -0700, David Vernet wrote:
> test_memcg_sock() in the cgroup memcg tests, verifies expected memory
> accounting for sockets. The test forks a process which functions as a TCP
> server, and sends large buffers back and forth between itself (as the TCP
> client) and the forked TCP server. While doing so, it verifies that
> memory.current and memory.stat.sock look correct.
> 
> There is currently a check in tcp_client() which asserts memory.current >=
> memory.stat.sock. This check is racy, as between memory.current and
> memory.stat.sock being queried, a packet could come in which causes
> mem_cgroup_charge_skmem() to be invoked. This could cause memory.stat.sock
> to exceed memory.current. Reversing the order of querying doesn't address
> the problem either, as memory may be reclaimed between the two calls.

But just curious, does it fix the flakiness (assuming there is no memory
pressure)?

> Instead, this patch just removes that assertion altogether, and instead
> relies on the values_close() check that follows to validate the expected
> accounting.

Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>