thp: fix MADV_DONTNEED vs. numa balancing race

	if (prot_numa && pmd_protnone(*pmd))

		goto unlock;

	entry = pmdp_huge_get_and_clear_notify(mm, addr, pmd);

	/*

	 * In case prot_numa, we are under down_read(mmap_sem). It's critical

	 * to not clear pmd intermittently to avoid race with MADV_DONTNEED

	 * which is also under down_read(mmap_sem):

	 *

	 *	CPU0:				CPU1:

	 *				change_huge_pmd(prot_numa=1)

	 *				 pmdp_huge_get_and_clear_notify()

	 * madvise_dontneed()

	 *  zap_pmd_range()

	 *   pmd_trans_huge(*pmd) == 0 (without ptl)

	 *   // skip the pmd

	 *				 set_pmd_at();

	 *				 // pmd is re-established

	 *

	 * The race makes MADV_DONTNEED miss the huge pmd and don't clear it

	 * which may break userspace.

	 *

	 * pmdp_invalidate() is required to make sure we don't miss

	 * dirty/young flags set by hardware.

	 */

	entry = *pmd;

	pmdp_invalidate(vma, addr, pmd);

	/*

	 * Recover dirty/young flags.  It relies on pmdp_invalidate to not

	 * corrupt them.

	 */

	if (pmd_dirty(*pmd))

		entry = pmd_mkdirty(entry);

	if (pmd_young(*pmd))

		entry = pmd_mkyoung(entry);

	entry = pmd_modify(entry, newprot);

	if (preserve_write)

		entry = pmd_mk_savedwrite(entry);