Re: [PATCH] mm/migrate: Fix NULL movable_ops if CONFIG_ZSMALLOC=m

[David Hildenbrand]

On 16.08.25 18:23, Matthew Wilcox wrote:
> On Sat, Aug 16, 2025 at 12:54:52PM +0200, David Hildenbrand wrote:
> > +++ b/mm/balloon_compaction.c
> > @@ -256,8 +256,10 @@ const struct movable_operations balloon_mops = {
> >   static int __init balloon_init(void)
> >   {
> > -	movable_ops[MOVABLE_BALLOON] = &balloon_mops;
> > -	return 0;
> > +	int rc;
> > +
> > +	rc = register_movable_ops(&balloon_mops, PGTY_offline);
> > +	return rc;
>
> Using 'rc' as the name of this variable is an anti-pattern.  All it
> tells you is "this is the return value".  Calling it 'err' is far
> better because now we know it's an error number (or zero for success,
> of course).

I know, we all have our things to complain about. Some about Cc: above 
--, others about the name of error variables :P

$ git grep "int rc" | wc -l
12730
$ git grep "int ret" | wc -l
80386
$ git grep "int error" | wc -l
4349
$ git grep "int err " | wc -l
6117

> It seems to be a particularly IBM derived antipattern ;-)

Careful miser :D

> Some internal style guide, perhaps?

Kernel-internal style guide maybe ;)

> > +void unregister_movable_ops(const struct movable_operations *ops, enum pagetype type)
> > +{
> > +	switch (type) {
> > +	case PGTY_offline:
> > +		WARN_ON_ONCE(offline_movable_ops != ops);
> > +		offline_movable_ops = NULL;
> > +		break;
> > +	case PGTY_zsmalloc:
> > +		WARN_ON_ONCE(zsmalloc_movable_ops != ops);
> > +		zsmalloc_movable_ops = NULL;
> > +		break;
>
> This might be a bit excessive ... just passing the pagetype and not
> having the sanity checks should be enough for the tiny number of users
> this interface will have.

Yeah, no strong opinion, this was a 3 minute hack.

--
Cheers

David / dhildenb