linux-kernel - Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan

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Date:   Sun, 16 Apr 2023 11:33:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Zach O'Keefe" <zokeefe@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>,
        Ivan Orlov <ivan.orlov0322@...il.com>,
        himadrispandya@...il.com, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org, linux-mm@...ck.org,
        skhan@...uxfoundation.org,
        syzbot+9578faa5475acb35fa50@...kaller.appspotmail.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in
 hpage_collapse_scan_file

Circling back to this fix...

The BUG() is obviously real.  We're unsure that Ivan's fix is the best
one.  We haven't identified a Fixes:, and as this report is against the 6.2
kernel, a cc:stable will be needed.

According to the sysbot bisection
(https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc),
this is present in linux-5.19, so it might predate Zach's
58ac9a8993a13ebc changes.  But that bisection claim might be
misleading.

And Zach is offline for a few months.  So can people please take a look
and see if we can get this wrapped up?

Matthew, the assertion failure is in the

	VM_BUG_ON(index != xas.xa_index);

which was added in 77da9389b9d5f, so perhaps you could take a look?

Thanks.