linux-kernel - Re: [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed

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Date:   Fri, 29 May 2020 13:37:13 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Qian Cai <cai@....pw>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>, andi.kleen@...el.com,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for
 vm_committed_as underflow check

On Thu, May 28, 2020 at 10:49:28PM -0400, Qian Cai wrote:
> On Fri, May 29, 2020 at 09:06:09AM +0800, Feng Tang wrote:
> > As is explained by Michal Hocko:
> > 
> > : Looking at the history, this has been added by 82f71ae4a2b8
> > : ("mm: catch memory commitment underflow") to have a safety check
> > : for issues which have been fixed. There doesn't seem to be any bug
> > : reports mentioning this splat since then so it is likely just
> > : spending cycles for a hot path (yes many people run with DEBUG_VM)
> > : without a strong reason.
> 
> Hmm, it looks like the warning is still useful to catch issues in,
> 
> https://lore.kernel.org/linux-mm/20140624201606.18273.44270.stgit@zurg
> https://lore.kernel.org/linux-mm/54BB9A32.7080703@oracle.com/
> 
> After read the whole discussion in that thread, I actually disagree with
> Michal. In order to get ride of this existing warning, it is rather
> someone needs a strong reason that could prove the performance hit is
> noticeable with some data.

One problem with current check is percpu_counter_read(&vm_committed_as)
is not accurate, and percpu_counter_sum() is way too heavy.

Thanks,
Feng