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Date:   Thu, 20 Oct 2016 16:23:32 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Jones <davej@...emonkey.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Chris Mason <clm@...com>, Jens Axboe <axboe@...com>,
        Al Viro <viro@...iv.linux.org.uk>, Josef Bacik <jbacik@...com>,
        David Sterba <dsterba@...e.com>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: bio linked list corruption.

On Thu, Oct 20, 2016 at 4:03 PM, Dave Jones <davej@...emonkey.org.uk> wrote:
> On Thu, Oct 20, 2016 at 04:01:12PM -0700, Andy Lutomirski wrote:
>  > On Thu, Oct 20, 2016 at 3:50 PM, Dave Jones <davej@...emonkey.org.uk> wrote:
>  > > On Tue, Oct 18, 2016 at 06:05:57PM -0700, Andy Lutomirski wrote:
>  > >
>  > >  > One possible debugging approach would be to change:
>  > >  >
>  > >  > #define NR_CACHED_STACKS 2
>  > >  >
>  > >  > to
>  > >  >
>  > >  > #define NR_CACHED_STACKS 0
>  > >  >
>  > >  > in kernel/fork.c and to set CONFIG_DEBUG_PAGEALLOC=y.  The latter will
>  > >  > force an immediate TLB flush after vfree.
>  > >
>  > > I can give that idea some runtime, but it sounds like this a case where
>  > > we're trying to prove a negative, and that'll just run and run ? In which case I
>  > > might do this when I'm travelling on Sunday.
>  >
>  > The idea is that the stack will be free and unmapped immediately upon
>  > process exit if configured like this so that bogus stack accesses (by
>  > the CPU, not DMA) would OOPS immediately.
>
> oh, misparsed. ok, I can definitely get behind that idea then.
> I'll do that next.
>

It could be worth trying this, too:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vmap_stack&id=174531fef4e8

It occurred to me that the current code is a little bit fragile.

--Andy

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