Re: "mm: fix lazyfree BUG_ON check in try_to_unmap_one()" build error

[Minchan Kim]

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@xxxxxxxxxx> 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.