Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole

[Peter Xu <peterx@redhat.com>]

On Tue, Aug 05, 2025 at 04:41:18PM -0700, Suren Baghdasaryan wrote:
> Ok, I let the reproducer run for half a day and it did not hit this
> case, so I must have done something wrong during my initial
> investigation. Sorry for the confusion. I could have sworn that I saw
> this case but now it just does not happen.

I'm wildly guessing you might have hit the numa balancing bug I mentioned,
that might explain what you mentioned previously on the testing results.
It might just be tricky to reproduce:

  - We'll need a valid THP (pmd) first in the MOVE source region

  - THP needs to be selected by numa balancing for a check (marking
    prot_none)

  - (before any further access..) UFFDIO_MOVE needs to happen on top trying
    to move the whole THP being marked as prot_none.

AFAICT, task_numa_work() is the only place that can mark the THP, and when
it happens, should see change_huge_pmd(cp_flags=MM_CP_PROT_NUMA) and then
returns with HPAGE_PMD_NR.

[sorry I am still pretty occupied with other things.  I can try to reproduce
 together with you after I get more time back]

> With migration entry being the only case that leads to that
> pmd_folio(), the only check we need to add is the "if
> (pmd_present(*src_pmd))" before pmd_folio(). Would you like me to
> check anything else or should I go ahead and post that fix?

We could fix the migration entry first, then if any of us can reproduce the
above numa balancing issue then it can be a 2nd patch on top.

After all, so far we didn't yet prove it, either some unreproduceable test,
or pure code analysis.  Meanwhile it might also be cleaner if we have one
patch fix one issue, rather than having one patch fix two bugs.

What do you think?

Thanks,

-- 
Peter Xu