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Date:   Sun, 16 Apr 2023 17:45:18 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Zach O'Keefe <zokeefe@google.com>, Yang Shi <shy828301@gmail.com>,
        Ivan Orlov <ivan.orlov0322@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>
Subject: Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in
 hpage_collapse_scan_file
In-Reply-To: <20230416113335.caeafda1328fdd4bfe2406e8@linux-foundation.org>
Message-ID: <f0fa9ad0-5077-e47a-8234-684f5cddcae0@google.com>
References: <20230330155305.423051-1-ivan.orlov0322@gmail.com> <20230331013301.ecgkjymaf3ws6rfb@google.com> <CAHbLzkoJ2zvKZpqd6NqikpCc8rGR02C87f0fPq8qwb1BF3K+9Q@mail.gmail.com> <CAAa6QmTLXSvygQQNk=8C4pB3D2Twy1guzo=+-aXhCqNAE3Q3MA@mail.gmail.com>
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On Sun, 16 Apr 2023, Andrew Morton wrote:

> 
> Circling back to this fix...
> 
> The BUG() is obviously real.

It's worth remembering that syzbot's reproducer involves artificially
injecting page allocation failures.  So although the bug may be "real",
it is rather in the theoretical category, way down on my own list to
look at, and I'd say not at all urgent to fix.

Hugh

> 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.