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

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Message-ID: <20200529024928.GA4566@lca.pw>
Date:   Thu, 28 May 2020 22:49:28 -0400
From:   Qian Cai <cai@....pw>
To:     Feng Tang <feng.tang@...el.com>
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 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.

> 
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> Cc: Konstantin Khlebnikov <koct9i@...il.com>
> Cc: Qian Cai <cai@....pw>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Andi Kleen <andi.kleen@...el.com>
> ---
>  mm/util.c | 8 --------
>  1 file changed, 8 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 3c7a08c..fe63271 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -814,14 +814,6 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>  {
>  	long allowed;
>  
> -	/*
> -	 * A transient decrease in the value is unlikely, so no need
> -	 * READ_ONCE() for vm_committed_as.count.
> -	 */
> -	VM_WARN_ONCE(data_race(percpu_counter_read(&vm_committed_as) <
> -			-(s64)vm_committed_as_batch * num_online_cpus()),
> -			"memory commitment underflow");
> -
>  	vm_acct_memory(pages);
>  
>  	/*
> -- 
> 2.7.4
>