linux-kernel - Re: kernel BUG at mm/shmem.c:LINE!

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Date:   Thu, 26 Jul 2018 12:32:03 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     syzbot <syzbot+b8e0dfee3fd8c9012771@...kaller.appspotmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: kernel BUG at mm/shmem.c:LINE!

On Thu, Jul 26, 2018 at 09:40:20AM -0700, Hugh Dickins wrote:
> On Thu, 26 Jul 2018, Matthew Wilcox wrote:
> > On Wed, Jul 25, 2018 at 11:53:15PM -0700, Hugh Dickins wrote:
> > 
> > and fixing the bug differently ;-)  But many thanks for spotting it!
> 
> I thought you might :)

The xas_* functions are all _expected_ to behave the same way when
passed an XA_STATE containing an error -- do nothing.  xas_create_range()
behaved that way initially, then I fixed a bug and broke that invariant.
Now the test suite checks it so I won't break it again.

> > I'll look into the next bug you reported ...
> 
> No need: that idea now works a lot better when I use the initialized
> "start", instead of the uninitialized "index".

Ugh.  xas_create_range() is _supposed_ to return with xas pointing to
the first index in the range.  I wonder what I messed up.  I've had a
go at producing a test-case for this and haven't provoked a bug yet.

Still, I don't want to keep xas_create_range() around long-term.
I want to transition all the places that currently use it to use
multi-index entries.  So I'm going to put your workaround in and then
work on deleting xas_create_range() altogether.

Thanks so much for all your work on this!