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

[Lorenzo Stoakes <ljs@kernel.org>]

On Sun, Jul 05, 2026 at 06:12:31AM -0700, 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.

Yikes!

>
> Pass src_pmd, matching __handle_mm_fault() and hmm_vma_walk_pmd().
>
> Reported-by: sashiko-bot <sashiko-bot@kernel.org>
> 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@linux.dev>

LGTM so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bdd8635922f9..c0892cc533a9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2775,7 +2775,7 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>  	if (!pmd_trans_huge(src_pmdval)) {
>  		spin_unlock(src_ptl);
>  		if (pmd_is_migration_entry(src_pmdval)) {
> -			pmd_migration_entry_wait(mm, &src_pmdval);
> +			pmd_migration_entry_wait(mm, src_pmd);
>  			return -EAGAIN;
>  		}
>  		return -ENOENT;
> --
> 2.53.0-Meta
>

Cheers, Lorenzo