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Date:   Mon, 17 Apr 2023 03:04:59 +0400
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Subject: Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in
 hpage_collapse_scan_file
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To:     Andrew Morton <akpm@linux-foundation.org>,
        Zach O'Keefe <zokeefe@google.com>
Cc:     Yang Shi <shy828301@gmail.com>, himadrispandya@gmail.com,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org, linux-mm@kvack.org,
        skhan@linuxfoundation.org,
        syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>
References: <20230330155305.423051-1-ivan.orlov0322@gmail.com>
 <20230331013301.ecgkjymaf3ws6rfb@google.com>
 <CAHbLzkoJ2zvKZpqd6NqikpCc8rGR02C87f0fPq8qwb1BF3K+9Q@mail.gmail.com>
 <CAAa6QmTLXSvygQQNk=8C4pB3D2Twy1guzo=+-aXhCqNAE3Q3MA@mail.gmail.com>
 <20230416113335.caeafda1328fdd4bfe2406e8@linux-foundation.org>
From:   Ivan Orlov <ivan.orlov0322@gmail.com>
In-Reply-To: <20230416113335.caeafda1328fdd4bfe2406e8@linux-foundation.org>
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On 4/16/23 22:33, Andrew Morton wrote:
> 
> 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.

I tested the reproducers on the 99cb0dbd47a15 commit, and they do not 
trigger the problematic condition of shared memory truncation or 
hole-punching. I will investigate further, as there have been many 
changes in khugepaged since the 99cb0dbd47a15 commit that could 
potentially affect its behavior.