Re: [PATCH] mm, memcg: fix error return value of mem_cgroup_alloc()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 06, 2020 at 10:16:56PM +0800, Yafang Shao wrote:
> On Mon, Apr 6, 2020 at 10:11 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Mon, Apr 06, 2020 at 10:09:55PM +0800, Yafang Shao wrote:
> > > What about ida_alloc_range() ? Should we correct it as well ?
> > >
> > >
> > > * Return: The allocated ID, or %-ENOMEM if memory could not be allocated,
> > > * or %-ENOSPC if there are no free IDs.
> > > */
> > > int ida_alloc_range(struct ida *ida, unsigned int min, unsigned int max,
> > >                         gfp_t gfp)
> > >
> > > If there're no free IDs, ida_alloc_range() also returns ENOSPC.
> >
> > Do you want to check all the callers of ida_.*alloc()?
> 
> Sorry, I can't your point.

Changing the value returned by ida_alloc_range() would require checking
every caller to see if it would break anything.  That's why I didn't
change it when I rewrote it.

> I just find the mem_cgroup_css_alloc() will use ida too.
> mem_cgroup_css_alloc
>     memcg_online_kmem
>         memcg_alloc_cache_id
>              ida_simple_get
> 
> I think we should keep the behavior consistent here.

ENOSPC doesn't make much sense here.  So I'd do:

	id = ida_simple_get(&memcg_cache_ida,
                            0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
+	if (id == -ENOSPC)
+		return -EBUSY;
	if (id < 0)
		return id;