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

[Rik van Riel]

On Sun, 2026-07-05 at 06:12 -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.
> 
> 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>

Reviewed-by: Rik van Riel <riel@xxxxxxxxxxx>

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