Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk_vmas

[David Hildenbrand]

On 17.06.25 10:50, Lorenzo Stoakes wrote:
> On Tue, Jun 17, 2025 at 10:38:07AM +0200, David Hildenbrand wrote:
> > > > > > 	vma = vma_modify_flags_name(&vmi, *prev, ...
> > > > > >
> > > > > > We should use Fixes: then.
> > > > >
> > > > > So no we shouldn't...
> > > > >
> > > > > > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > >
> > > > > Sure? :)
> > > >
> > > > Unless I am missing something important, yes :)
> > >
> > > This solution isn't correct as prev == NULL when prev != NULL is wholly
> > > incorrect.
> >
> > I am not able to understand what you mean :)
> >
> > I assume you mean, that we reach a point down in the callchain, where "prev"
> > is supposed to be set to something proper, but it would be "NULL".
>
> I mean if you tell merge code 'hey the previous VMA is NULL' (same thing as
> saying 'hey this is the first VMA in the address space) and it isn't, bad things
> will happen (TM).
>
> > That would indeed require a different fix.
>
> Yes this patch is wrong, sorry.
>
> > I wonder why we didn't trigger this case so far?
>
> It's because it only happens since Barry's per-VMA lock logic...
>
> 	if (madv_behavior && madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) {
> 		vma = try_vma_read_lock(mm, madv_behavior, start, end);
> 		if (vma) {
> 			error = visit(vma, &prev, start, end, arg);
> 			vma_end_read(vma);
> 			return error;
> 		}
> 	}
>
> Otherwise, we look up the find_vma_prev():
>
> 	vma = find_vma_prev(mm, start, &prev);
>
> In madvise_dontneed_free() we always set *prev = vma _first_.
>
> Let me suggest the better fix to Lance higher in thread so he sees :)
>
> Not sure if a fixes is valid here given this isn't mainline yet, more so this
> should be squashed with barry's series?

If it's not in mm-stable yet, it can still be squashed, yes.

--
Cheers,

David / dhildenb