Re: [PATCH] userfaultfd: wait on source PMD during UFFDIO_MOVE

[Baolin Wang]

On 7/5/26 9:12 PM, Usama Arif wrote:
> move_pages_huge_pmd() snapshots src_pmdval under src_ptl, drops the lock,
> and, for migration entries, waits with pmd_migration_entry_wait().
>
> Passing &src_pmdval is wrong. pmd_migration_entry_wait() must lock and
> re-read the real page-table PMD; on split-PMD-lock kernels, a stack
> address also resolves to the wrong lock. softleaf_entry_wait_on_locked()
> then waits without a folio reference, which is safe only while serialized
> against migration-entry removal by the real PT lock.
>
> Pass src_pmd, matching __handle_mm_fault() and hmm_vma_walk_pmd().
>
> Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=8
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---

Good catch. Agree with Andrew that we should cc stable.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>