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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check
Date:   Wed, 3 Jun 2020 07:51:37 -0400
Message-Id: <B26ADCE5-796F-46C6-8445-2D877B881B58@lca.pw>
References: <20200603094804.GB89848@shbuild999.sh.intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Kees Cook <keescook@chromium.org>, andi.kleen@intel.com,
        tim.c.chen@intel.com, dave.hansen@intel.com, ying.huang@intel.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Konstantin Khlebnikov <koct9i@gmail.com>
In-Reply-To: <20200603094804.GB89848@shbuild999.sh.intel.com>
To:     Feng Tang <feng.tang@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 3, 2020, at 5:48 AM, Feng Tang <feng.tang@intel.com> wrote:
>=20
> This check was added by 82f71ae4a2b8 ("mm: catch memory commitment underfl=
ow")
> in 2014 to have a safety check for issues which have been fixed.
> And there has been few report caught by it, as described in its
> commit log:
>=20
> : This shouldn't happen any more - the previous two patches fixed
> : the committed_as underflow issues.
>=20
> But it was really found by Qian Cai when he used the LTP memory
> stress suite to test a RFC patchset, which tries to improve scalability
> of per-cpu counter 'vm_committed_as', by chosing a bigger 'batch' number
> for loose overcommit policies (OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS),
> while keeping current number for OVERCOMMIT_NEVER.
>=20
> With that patchset, when system firstly uses a loose policy, the
> 'vm_committed_as' count could be a big negative value, as its big 'batch'
> number allows a big deviation, then when the policy is changed to
> OVERCOMMIT_NEVER, the 'batch' will be decreased to a much smaller value,
> thus hits this WARN check.
>=20
> To mitigate this, one proposed solution is to queue work on all online
> CPUs to do a local sync for 'vm_committed_as' when changing policy to
> OVERCOMMIT_NEVER, plus some global syncing to garante the case won't
> be hit.
>=20
> But this solution is costy and slow, given this check hasn't shown real
> trouble or benefit, simply drop it from one hot path of MM. And perf
> stats does show some tiny saving for removing it.

The text looks more reasonable than the previous one.

Reviewed-by: Qian Cai <cai@lca.pw>=