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

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Message-ID: <Yqf+PC+cKePAsaNI@google.com>
Date:   Tue, 14 Jun 2022 12:19:24 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        umgwanakikbuti@...il.com, bigeasy@...utronix.de,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        regressions@...ts.linux.dev, Jens Axboe <axboe@...nel.dk>,
        Nitin Gupta <ngupta@...are.org>
Subject: Re: qemu-arm: zram: mkfs.ext4 : Unable to handle kernel NULL pointer
 dereference at virtual address 00000140

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.