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.

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.