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SubjectRe: [BUG] ext4/block null pointer crashes in linux-next
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On Tue, 16 Oct 2018 08:42:14 -0400, valdis.kletnieks@vt.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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