mm: mlock: avoid folio_within_range() on KSM pages

Since commit dc68badc ("mm: mlock: update mlock_pte_range to handle
large folio") I've just occasionally seen VM_WARN_ON_FOLIO(folio_test_ksm)
warnings from folio_within_range(), in a splurge after testing with KSM
hyperactive.

folio_referenced_one()'s use of folio_within_vma() is safe because it
checks folio_test_large() first; but allow_mlock_munlock() needs to do the
same to avoid those warnings (or check !folio_test_ksm() itself?  Or move
either check into folio_within_range()?  Hard to tell without more
examples of its use).

Link: https://lkml.kernel.org/r/23852f6a-5bfa-1ffd-30db-30c5560ad426@google.com
Fixes: dc68badc ("mm: mlock: update mlock_pte_range to handle large folio")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mlock.c

@@ -346,6 +346,10 @@ static inline bool allow_mlock_munlock(struct folio *folio,
 	if (!(vma->vm_flags & VM_LOCKED))
 		return true;
 
+	/* folio_within_range() cannot take KSM, but any small folio is OK */
+	if (!folio_test_large(folio))
+		return true;
+
 	/* folio not in range [start, end), skip mlock */
 	if (!folio_within_range(folio, vma, start, end))
 		return false;