kernel: bio too big device md6 (248 > 240)

https://bugzilla.redhat.com/show_bug.cgi?id=498162


dmsetup reload luks-md2 --table "`dmsetup table --showkeys luks-md2`" dmsetup suspend luks-md2; dmsetup resume luks-md2 Does appear to fix the issue - although it's pretty scary (md2 is the root fs, so this only works because of caching).

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https://bugzilla.kernel.org/show_bug.cgi?id=9401  as well

Also if crypt device mapping table is reloaded (to force new restriction to
apply),
it will work correctly:

echo 3 > /proc/sys/vm/drop_caches
dmsetup suspend crypt-punchbag
dmsetup reload crypt-punchbag --table "`dmsetup table --showkeys
crypt-punchbag`"
dmsetup resume crypt-punchbag
sha1sum /mnt/punchbag/junk

Problem is now fully understood but solution is not so simple.
This problem can happen with arbitrary block devices stacked mapping.