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Date:   Thu, 26 Jul 2018 07:33:53 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     syzbot <syzbot+b8e0dfee3fd8c9012771@syzkaller.appspotmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: kernel BUG at mm/shmem.c:LINE!
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On Wed, Jul 25, 2018 at 11:53:15PM -0700, Hugh Dickins wrote:
> Now I've learnt that an oops on 0xffffffffffffffbe points to EEXIST,
> not to EREMOTE, it's easy: patch below fixes those four xfstests
> (and no doubt a similar oops I've seen occasionally under swapping
> load): so gives clean xfstests runs for non-huge and huge tmpfs.

Excellent!

I'm adding this:

+++ b/lib/test_xarray.c
@@ -741,6 +741,13 @@ static noinline void check_create_range_2(struct xarray *xa
, unsigned order)
        XA_BUG_ON(xa, !xa_empty(xa));
 }
 
+static noinline void check_create_range_3(void)
+{
+       XA_STATE(xas, NULL, 0);
+       xas_set_err(&xas, -EEXIST);
+       xas_create_range(&xas);
+}
+
 static noinline void check_create_range(struct xarray *xa)
 {
        unsigned int order;
@@ -755,6 +762,8 @@ static noinline void check_create_range(struct xarray *xa)
                if (order < 10)
                        check_create_range_2(xa, order);
        }
+
+       check_create_range_3();
 }
 
 static LIST_HEAD(shadow_nodes);

and fixing the bug differently ;-)  But many thanks for spotting it!

I'll look into the next bug you reported ...