linux-kernel - Re: "mm: fix lazyfree BUG_ON check in try_to_unmap

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Date:   Fri, 10 Mar 2017 09:45:22 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: "mm: fix lazyfree BUG_ON check in try_to_unmap_one()" build error

Hi Andrew,

On Thu, Mar 09, 2017 at 01:27:06PM -0800, Andrew Morton wrote:
> On Thu, 9 Mar 2017 15:02:26 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
> > Sergey reported VM_WARN_ON_ONCE returns void with !CONFIG_DEBUG_VM
> > so we cannot use it as if's condition unlike WARN_ON.
> 
> Can we instead fix VM_WARN_ON_ONCE()?

I thought the direction but the reason to decide WARN_ON_ONCE in this case
is losing of benefit with using CONFIG_DEBU_VM if we go that way.

I think the benefit with VM_WARN_ON friends is that it should be completely
out from the binary in !CONFIG_DEBUG_VM. However, if we fix VM_WARN_ON
like WARN_ON to !!condition, at least, compiler should generate condition
check and return so it's not what CONFIG_DEBUG_VM want, IMHO.
However, if guys believe it's okay to add some instructions to debug VM
although we disable CONFIG_DEBUG_VM, we can go that way.
It's a just policy matter. ;-)

Anyway, Even though we fix VM_WARN_ON_ONCE, in my case, WARN_ON_ONCE is
better because we should do !!condition regardless of CONFIG_DEBUG_VM
and if so, WARN_ON is more wide coverage than VM_WARN_ON which only works
with CONFIG_DEBUG_VM.

Thanks.