mm/memory: handle non-split locks correctly in zap_empty_pte_table()

While we handle pte_lockptr() == pmd_lockptr() correctly in
zap_pte_table_if_empty(), we don't handle it in zap_empty_pte_table(),
making the spin_trylock() always fail and forcing us onto the slow path.

So let's handle the scenario where pte_lockptr() == pmd_lockptr() better,
which can only happen if CONFIG_SPLIT_PTE_PTLOCKS is not set.

This is only relevant once we unlock CONFIG_PT_RECLAIM on architectures
that are not x86-64.

Link: https://lkml.kernel.org/r/20260119220708.3438514-3-david@kernel.org
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: David Hildenbrand (Red Hat)

mm/memory.c

@@ -1830,16 +1830,18 @@ static bool pte_table_reclaim_possible(unsigned long start, unsigned long end,
 	return details && details->reclaim_pt && (end - start >= PMD_SIZE);
 }
 
-static bool zap_empty_pte_table(struct mm_struct *mm, pmd_t *pmd, pmd_t *pmdval)
+static bool zap_empty_pte_table(struct mm_struct *mm, pmd_t *pmd,
+		spinlock_t *ptl, pmd_t *pmdval)
 {
 	spinlock_t *pml = pmd_lockptr(mm, pmd);
 
-	if (!spin_trylock(pml))
+	if (ptl != pml && !spin_trylock(pml))
 		return false;
 
 	*pmdval = pmdp_get(pmd);
 	pmd_clear(pmd);
-	spin_unlock(pml);
+	if (ptl != pml)
+		spin_unlock(pml);
 	return true;
 }
 
@@ -1931,7 +1933,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 	 * from being repopulated by another thread.
 	 */
 	if (can_reclaim_pt && direct_reclaim && addr == end)
-		direct_reclaim = zap_empty_pte_table(mm, pmd, &pmdval);
+		direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval);
 
 	add_mm_rss_vec(mm, rss);
 	lazy_mmu_mode_disable();