Re: [PATCH] memcg: remove unused mem_cgroup_from_obj()

[Yosry Ahmed <yosryahmed@google.com>]

On Sat, May 27, 2023 at 8:07 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, May 26, 2023 at 09:13:05PM -0700, Yosry Ahmed wrote:
> > On Fri, May 26, 2023 at 9:01 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Sat, May 27, 2023 at 06:31:26PM +0800, Miaohe Lin wrote:
> > > > The function mem_cgroup_from_obj() is not used anymore. Remove it and
> > > > clean up relevant comments.
> > >
> > > You should have looked at the git history to see why it was created
> > > and who used it.
> > >
> > > Shakeel, Vasily, are you going to retry adding commit 1d0403d20f6c?
> >
> > That commit did not introduce the function though, no? It was
> > introduced before it and replaced by other variants over time (like
> > mem_cgroup_from_slab_obj()). It looks like that commit was reverted ~9
> > months ago. We can always bring it back if/when needed.
>
> The commit immediately preceding it is fc4db90fe71e.
>
> Of course we can bring it back.  It's just code.  But avoiding
> unnecessary churn is also good.  Let's wait to hear from Vasily.
>
> > It also looks to me that 1d0403d20f6c was using mem_cgroup_from_obj()
> > on a struct net object, which is allocated in net_alloc() from a slab
> > cache, so mem_cgroup_from_slab_obj() should be sufficient, no?
>
> Clearly not.

I dived deeper into the history on LKML, and you are right:
https://lore.kernel.org/all/Yp4F6n2Ie32re7Ed@qian/

I still do not understand why mem_cgroup_from_slab_obj() would not be
sufficient, so I am hoping Vasily or Shakeel can help me understand
here. Seems to be something arch-specific.

Thanks for digging this up, Matthew.