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Date:   Mon, 9 Apr 2018 18:18:16 +0200 (CEST)
From:   Sebastian Ott <sebott@linux.ibm.com>
To:     Kees Cook <keescook@chromium.org>
cc:     Sebastian Ott <sebott@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Martin Uecker <Martin.Uecker@med.uni-goettingen.de>,
        Ingo Molnar <mingo@kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [bisected] 3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91 oopses on
 s390
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On Fri, 6 Apr 2018, Kees Cook wrote:
> On Fri, Apr 6, 2018 at 2:47 AM, Sebastian Ott <sebott@linux.vnet.ibm.com> wrote:
> > Today's kernel oopsed on s390. Bisect points to:
> > 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
> >
> > [    1.898277] dasd-eckd 0.0.3304: DASD with 4 KB/block, 21636720 KB total size, 48 KB/track, compatible disk layout
> > [    1.898308] ------------[ cut here ]------------
> > [    1.898310] kernel BUG at block/bio.c:1798!
> 
> Well that's extremely bad. :(

What happened is that the bio build by the partition detection code was
attempted to be split by the block layer because the block queue had a
max_sector setting of 0. blk_queue_max_hw_sectors uses min_not_zero.

Both of the following return 0 on my machine:
+       pr_warn("%u\n", min_not_zero(100, 1000));
+       pr_warn("%u\n", min_not_zero(1000, 100));

So, we now know what failed...the question is why?

Sebastian