Re: [akpm-mm:mm-new] BUILD SUCCESS WITH UNVERIFIED WARNING e0ffd277b09a7c9f1a436e5f75be37ac490bc59f

[Stanislav Kinsburskii <skinsburskii@gmail.com>]

On Mon, Aug 10, 2026 at 01:23:34PM -0700, Andrew Morton wrote:
> On Mon, 10 Aug 2026 10:44:13 +0800 kernel test robot <lkp@intel.com> wrote:
> 
> > tree/branch: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-new
> > branch HEAD: e0ffd277b09a7c9f1a436e5f75be37ac490bc59f  mm/swap, PM: hibernate: atomically replace hibernation pin
> > 
> > Unverified Warning (likely false positive, kindly check if interested):
> > 
> >     https://lore.kernel.org/oe-kbuild/202607120402.clBrrZQA-lkp@intel.com
> > 
> >     mm/hmm.c:673 hmm_do_fault() error: we previously assumed 'hmm_vma_walk->locked' could be null (see line 654)
> > 
> > Warning ids grouped by kconfigs:
> 
> Thanks.  I assume this warning was produced by sparse?  The report
> doesn't tell us this, and I suggest that it do so.
> 
> > recent_errors
> > `-- s390-randconfig-r071-20260809
> >     `-- mm-hmm.c-hmm_do_fault()-error:we-previously-assumed-hmm_vma_walk-locked-could-be-null-(see-line-)
> 
> Caused by
> 
> 	121170831228 ("mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support")
> 
> 
> Stanislaw, you've previously said that this is a can't-happen, but boy
> I'm getting tired of seeing this warning report.  Would the below change
> hurt anything?  I can't immediately think of a cost-free way of doing
> this.
> 

I think this change doesn't do any harm.
Perhaps it worth a WARN_ON_ONCE to expose a wrong fault handler
behavior?

Thanks,
Stanislav

> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm/hmm.c:hmm_do_fault(): suppress sparse warning
> Date: Mon Aug 10 01:16:37 PM PDT 2026
> 
> mm/hmm.c:673 hmm_do_fault() error: we previously assumed 'hmm_vma_walk->locked' could be null (see line 654)
> 
> Stanislav says this can't happen.  Waste a few cycles to make the warning
> go away.
> 
> Fixes: 121170831228 ("mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/202608101053.PhnVUM4u-lkp@intel.com
> Cc: Stanislav Kinsburskii <skinsburskii@gmail.com>
> Cc: David Hildenbrand <david@kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/hmm.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> --- a/mm/hmm.c~mm-hmmc-hmm_do_fault-suppress-sparse-warning
> +++ a/mm/hmm.c
> @@ -670,7 +670,8 @@ static int hmm_do_fault(struct mm_struct
>  		ret = handle_mm_fault(vma, addr, fault_flags, NULL);
>  
>  		if (ret & (VM_FAULT_COMPLETED | VM_FAULT_RETRY)) {
> -			*hmm_vma_walk->locked = false;
> +			if (hmm_vma_walk->locked)	/* needed by sparse */
> +				*hmm_vma_walk->locked = false;
>  			return HMM_FAULT_UNLOCKED;
>  		}
>  
> _
>