Re: [syzbot] [mm?] kernel BUG in vma_replace_policy

[Hugh Dickins <hughd@google.com>]

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On Fri, 15 Sep 2023, Suren Baghdasaryan wrote:
> On Fri, Sep 15, 2023 at 9:09 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > Thanks for the feedback, Hugh!
> > Yeah, this positive err handling is kinda weird. If this behavior (do
> > as much as possible even if we fail eventually) is specific to mbind()
> > then we could keep walk_page_range() as is and lock the VMAs inside
> > the loop that calls mbind_range() with a condition that ret is
> > positive. That would be the simplest solution IMHO. But if we expect
> > walk_page_range() to always apply requested page_walk_lock policy to
> > all VMAs even if some mm_walk_ops returns a positive error somewhere
> > in the middle of the walk then my fix would work for that. So, to me
> > the important question is how we want walk_page_range() to behave in
> > these conditions. I think we should answer that first and document
> > that. Then the fix will be easy.
> 
> I looked at all the cases where we perform page walk while locking
> VMAs and mbind() seems to be the only one that would require
> walk_page_range() to lock all VMAs even for a failed walk.

Yes, I can well believe that.

> So, I suggest this fix instead and I can also document that if
> walk_page_range() fails it might not apply page_walk_lock policy to
> the VMAs.
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 42b5567e3773..cbc584e9b6ca 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1342,6 +1342,9 @@ static long do_mbind(unsigned long start,
> unsigned long len,
>          vma_iter_init(&vmi, mm, start);
>          prev = vma_prev(&vmi);
>          for_each_vma_range(vmi, vma, end) {
> +                /* If queue_pages_range failed then not all VMAs
> might be locked */
> +                if (ret)
> +                        vma_start_write(vma);
>                  err = mbind_range(&vmi, vma, &prev, start, end, new);
>                  if (err)
>                          break;
> 
> If this looks good I'll post the patch. Matthew, Hugh, anyone else?

Yes, I do prefer this, to adding those pos ret mods into the generic
pagewalk.  The "if (ret)" above being just a minor optimization, that
I would probably not have bothered with (does it even save any atomics?)
- but I guess it helps as documentation.

I think it's quite likely that mbind() will be changed sooner or later
not to need this; but it's much the best to fix this vma locking issue
urgently as above, without depending on any mbind() behavioral discussions.

Thanks,
Hugh