Re: [PATCH] mm: fs: fix lru_cache_disabled race in bh_lru

[Minchan Kim]

On Wed, Mar 09, 2022 at 02:06:27PM -0800, Andrew Morton wrote:
> On Tue,  8 Mar 2022 10:07:09 -0800 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> > Check lru_cache_disabled under bh_lru_lock. Otherwise, it could
> > introduce race below and it fails to migrate pages containing
> > buffer_head.
> > 
> >    CPU 0					CPU 1
> > 
> > bh_lru_install
> >                                        lru_cache_disable
> >   lru_cache_disabled = false
> >                                        atomic_inc(&lru_disable_count);
> > 				       invalidate_bh_lrus_cpu of CPU 0
> > 				       bh_lru_lock
> > 				       __invalidate_bh_lrus
> > 				       bh_lru_unlock
> >   bh_lru_lock
> >   install the bh
> >   bh_lru_unlock
> 
> What are the user-visible runtime effects of this bug?

Once the race happens, CMA allocation fails, which is critical for
the workload CMA allocation depends.

> 
> Is a cc:stable needed?

Ah, missed it. I think it would be rare to trigger the race considering
how CMA allocation would be rare but once it happens, it makes the CMA
allocation failure, which is critical for some. And the patch size is
small enough so I think it's worth to add in the stable.

> 
> Should there be a reported-by?

I found it on my own while I reviewed Marcelo's other patchset so
I don't think we need to add my reported-by.

Andrew, please tell me if you want me resend it.