Re: [PATCH] mm/page_isolation: don't putback unisolated page

[John Hubbard]

On 9/9/21 1:56 AM, David Hildenbrand wrote:
> On 09.09.21 00:42, John Hubbard wrote:
> > On 9/7/21 2:56 AM, David Hildenbrand wrote:
> > ...
> > > > If this can be handled gracefully, then I'd rather go with VM_WARN_ON.
> > > > Maybe even WARN_ON_ONCE?
> > >
> > > I think either VM_BUG_ON() or VM_WARN_ON() -- compiling the runtime checks out -- should be good
> > > enough.
> > >
> > > I'd just go with VM_BUG_ON(), because anybody messing with __isolate_free_page() should clearly spot
> > > that we expect the current handling. But no strong opinion.
> >
> > If in doubt, WARN*() should be preferred over BUG*(). There's a pretty long
> > history of "don't kill the machine unless you have to" emails about this, let
> > me dig up one...OK, maybe not the best example, but the tip of the iceberg:
>
> Please note the subtle difference between BUG_ON and VM_BUG_ON. We expect VM_BUG_ON to be compiled 
> out on any production system. So it's really only a mean to identify things that really shouldn't be 
> like that during debugging/testing.

Yes, but the end result is the same: it halts the system. It don't think it changes
the reasoning about BUG vs WARN very much.


> Using WARN... instead of VM_BUG_ON is even worse for production systems. There are distros that set 
> panic_on_warn, essentially converting WARN... into BUG...

This is different than BUG: panic() *prints a backtrace*, and then reboots the system.
That is still awkward, but a little more debuggable.

thanks,
--
John Hubbard
NVIDIA