Re: [PATCH 1/9] memcg: make the v1 soft limit knob inert

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Aug 12, 2026 at 03:50:10PM -0700, Andrew Morton wrote:
> On Tue, 11 Aug 2026 13:31:55 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:
> 
> > The v1 soft limit has been deprecated since v6.12 and nobody has
> > reported depending on it. Start the removal by decoupling the interface
> > from the implementation: keep memory.soft_limit_in_bytes, but ignore
> > writes to it and always report the maximum value on read similar to
> > what memory.kmem.limit_in_bytes already does.
> > 
> > Writes are still parsed, so malformed input keeps returning -EINVAL.
> > The knob now also behaves the same everywhere: it used to return
> > -EOPNOTSUPP on PREEMPT_RT, where soft limit reclaim has always been
> > disabled.
> > 
> > This also fixes the syzbot report linked below. Soft limit reclaim is
> > the only caller that runs shrink_lruvec() from kswapd against a
> > specific memcg, so it is the only way to reach lru_gen_shrink_lruvec()
> > and in turn set_mm_walk(), which warns when called from kswapd.
> 
> Thanks, I'll pluck this one as a hotfix.

Sounds good.

> 
> It's old code, yes?

Yes.

> Are there any theories about why the sysbot issue
> has just popped up?

The reproducer is toggling MGLRU on and off continuosly which is not really
recommended.


> Should we backport the fix?

I have no concerns in backporting this to older stable kernels.