linux-kernel - Re: [BUG] ext4/block null pointer crashes in linux-next

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Message-ID: <12638.1539706333@turing-police.cc.vt.edu>
Date:   Tue, 16 Oct 2018 12:12:13 -0400
From:   valdis.kletnieks@...edu
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Dennis Zhou <dennisszhou@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [BUG] ext4/block null pointer crashes in linux-next

On Tue, 16 Oct 2018 08:42:14 -0400, valdis.kletnieks@...edu said:

> [~] zgrep CGROUP /proc/config.gz
> CONFIG_CGROUPS=y
> CONFIG_BLK_CGROUP=y
> # CONFIG_DEBUG_BLK_CGROUP is not set
> (...)

> Looks like I should enable DEBUG_BLK_CGROUP and see what that says.

Built next-20181016 with that enabled - and the exact same kablam.

Started looking at the code, and I noticed that if the .config has

CONFIG_IOSCHED_CFQ=y
# CONFIG_CFQ_GROUP_IOSCHED is not set
CONFIG_BLK_CGROUP=y

then the 'struct cfqg_stats' at line 185 of block/cfq-iosched.c ends up being
zero length (which hints that combo of config variables is a pathological
case). I haven't delved through all the #ifdef'ery, but I wonder if there
may be other corner-case issues with that combo....


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