Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file

[Ivan Orlov]

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.

Hello, Andrew!

I have been unable to reproduce the problem with any of the existing 
reproducers on the 3d7cb6b04c3f3115719235cc6866b10326de34cd commit, 
which was detected by the syzkaller bisection. I also tried to test if 
the problem is reproducible on this commit via syzbot, but it did not 
detect the problem. It's possible that the bisection claim is 
misleading, as the issue may not be consistently reproducible.

Why did you mention the 58ac9a8993a13ebc commit? I thought 99cb0dbd47a15 
was considered as a "Fixes". 99cb0dbd47a15 is older than 3d7cb6b04c3f3, 
and the problematic code appeared there, so probably the problem could 
appear in 99cb0dbd47a15 as well.

Please, correct me if I'm missing something.

Kind regards,
Ivan Orlov.