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

[Andrew Morton <akpm@linux-foundation.org>]

On Sun,  5 Jul 2026 06:12:31 -0700 Usama Arif <usama.arif@linux.dev> 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().

Thanks.

> Reported-by: sashiko-bot <sashiko-bot@kernel.org>
> Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=8

Sashiko says "wild spinlock acquisition and memory corruption", so I'll
add a cc:stable to this.

> --- 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;