Re: Regression in 4.14: wrong data being read from bcache device

Jens, Christoph & everyone--

Hi-- I believe I have a general idea what's going on.

On Thu, Nov 16, 2017 at 3:02 PM, Michael Lyle [off-list ref] wrote:

On 11/16/2017 12:31 PM, Jens Axboe wrote:

quoted

I'd suggest just doing a bisect between 4.13 and 4.14, sounds like the
issue is trivially reproducible.

Took me awhile to get a repro but I have one now.  Will start bisecting
tonight.

It looks like the regression came from commit is
74d46992e0d9dee7f1f376de0d56d31614c8a17a (3/3 repros with it, 0/3
repros without it and hacked to build).

I don't have time to analyze this tonight as I have a couple other
things to do, but will look at fully analyzing it tomorrow morning.

I think the probable cause is this: we construct bios based on
previously completed bios in a few places, but blk_partition_remap
sets the partno to 0 and adjusts the disk offset.  It seems like a lot
of potential uses of bio_copy_dev could be dangerous for this reason.

Mike