Re: [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check

[Qian Cai <cai@lca.pw>]

> On Jun 3, 2020, at 5:48 AM, Feng Tang <feng.tang@intel.com> wrote:
> 
> This check was added by 82f71ae4a2b8 ("mm: catch memory commitment underflow")
> 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:
> 
> : This shouldn't happen any more - the previous two patches fixed
> : the committed_as underflow issues.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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>