Re: qemu-arm: zram: mkfs.ext4 : Unable to handle kernel NULL pointer dereference at virtual address 00000140

[Minchan Kim <minchan@kernel.org>]

On Tue, Jun 14, 2022 at 12:19:24PM +0900, Sergey Senozhatsky wrote:
> On (22/06/13 09:49), Minchan Kim wrote:
> > > Many thanks for the tests.
> > > 
> > > Quite honestly I was hoping that the patch would not help :) Well, ok,
> > > we now know that it's mapping area lock and the lockdep part of its
> > > memory is zero-ed out. The question is - "why?" It really should not
> > > be zeroed out.
> > 
> > Ccing Mike and Sebastian who are author/expert of the culprit patch
> > 
> > Naresh found zsmalloc crashed on the testing [1] and confirmed
> > that Sergey's patch[2] fixed the problem.
> > However, I don't understand why we need reinit the local_lock
> > on cpu_up handler[3].
> > 
> > Could you guys shed some light?
> 
> My guess is that it's either something very specific to Naresh's arch/config
> or a bug somewhere, which memset() per-CPU memory. Not sure how to track it
> down. KASAN maybe?
> 
> We certainly don't expect that
> 
> 	static DEFINE_PER_CPU(struct mapping_area, zs_map_area) = {
> 	        .lock   = INIT_LOCAL_LOCK(lock),
> 	};
> 
> would produce un-initialized dep_map. So I guess we start off with a
> valid per-CPU lock, but then it somehow gets zeroed-out.

Yes, I don't think we need to reinitialize the local_lock.

Naresh, we believe the patch Sergey provided for the test
was just band aid to hide the problem.

Could you please try to bisect it?