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

[Feng Tang]

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@xxxxxxxxxx/
> 
> 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