mm/memcontrol: Fix LRU reparenting logic when !MGLRU

The Multi-Gen LRU (MGLRU) is enabled by default via
CONFIG_LRU_GEN_ENABLED=y, but it can be dynamically disabled by writing
n to /sys/kernel/mm/lru_gen/enabled. While attempting to disable the
MGLRU and switching over to the classic LRU (anon/file), the following
crash is seen:

>> trace
 #0  crash_setup_regs (./arch/x86/include/asm/kexec.h:111:15)
 #1  __crash_kexec (kernel/crash_core.c:122:4)
 #2  panic (kernel/panic.c:399:3)
 #3  oops_end (arch/x86/kernel/dumpstack.c:382:3)
 #4  do_trap_no_signal (arch/x86/kernel/traps.c:156:3)
 #5  do_trap (arch/x86/kernel/traps.c:197:7)
 #6  do_error_trap (arch/x86/kernel/traps.c:217:3)
 #7  handle_invalid_op (arch/x86/kernel/traps.c:254:2)
 #8  exc_invalid_op (arch/x86/kernel/traps.c:314:2)
 #9  asm_exc_invalid_op+0x1a/0x1f (./arch/x86/include/asm/idtentry.h:621)
 #10 __list_del_entry_valid_or_report (lib/list_debug.c:62:6)
 #11 __list_del_entry_valid (./include/linux/list.h:124:9)
 #12 __list_del_entry (./include/linux/list.h:215:7)
 #13 list_del (./include/linux/list.h:229:2)
 #14 lruvec_del_folio (./include/linux/mm_inline.h:361:3)
 #15 lru_activate (mm/swap.c:342:2)
 #16 folio_batch_move_lru (mm/swap.c:199:3)
 #17 lru_add_drain (mm/swap.c:734:2)
 #18 wp_can_reuse_anon_folio (mm/memory.c:3788:3)
 #19 do_wp_page (mm/memory.c:3900:39)
 #20 __handle_mm_fault (mm/memory.c:6090:9)
 #21 handle_mm_fault (mm/memory.c:6258:9)
 #22 do_user_addr_fault (arch/x86/mm/fault.c:1338:10)
 #23 handle_page_fault (arch/x86/mm/fault.c:1481:3)
 #24 exc_page_fault (arch/x86/mm/fault.c:1539:2)
 #25 asm_exc_page_fault+0x26/0x2b (./arch/x86/include/asm/idtentry.h:623)
 #26 0x7f82f0896ed2

The issue occurs because the MGLRU-specific version of
lruvec_reparent_relocate() is invoked even when the system is using the
classic LRU. Fix this, by ensuring the call is properly gated by
lru_gen_enabled(), which checks whether MGLRU is enabled either
statically via Kconfig or dynamically via the /sys interface.

Orabug: 37920452

Fixes: cc65a5d ("mm: memcontrol: use obj_cgroup APIs to charge the LRU pages")
Cc: Tom Hromatka <[email protected]>
Reported-by: Sidhartha Kumar <[email protected]>
Reviewed-by: Sidhartha Kumar <[email protected]>
Reviewed-by: Harry Yoo <[email protected]>
Tested-by: Harry Yoo <[email protected]>
Reviewed-by: Imran Khan <[email protected]>
Signed-off-by: Kamalesh Babulal <[email protected]>
Signed-off-by: Harry Yoo <[email protected]>

Author: kamalesh-babulal

mm/memcontrol.c

@@ -244,11 +244,48 @@ static void lruvec_reparent_lock(struct mem_cgroup *src, struct mem_cgroup *dst)
         for ((gen) = 0; (gen) < MAX_NR_GENS; (gen)++)                   \
                 for ((type) = 0; (type) < ANON_AND_FILE; (type)++)      \
                         for ((zone) = 0; (zone) < MAX_NR_ZONES; (zone)++)
+#endif
+
+static void lruvec_reparent_lru(struct lruvec *src, struct lruvec *dst,
+				enum lru_list lru)
+{
+	int zid;
+	struct mem_cgroup_per_node *mz_src, *mz_dst;
+
+	mz_src = container_of(src, struct mem_cgroup_per_node, lruvec);
+	mz_dst = container_of(dst, struct mem_cgroup_per_node, lruvec);
+
+	if (lru != LRU_UNEVICTABLE)
+		list_splice_tail_init(&src->lists[lru], &dst->lists[lru]);
+
+	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
+		mz_dst->lru_zone_size[zid][lru] += mz_src->lru_zone_size[zid][lru];
+		mz_src->lru_zone_size[zid][lru] = 0;
+	}
+}
 
 static void lruvec_reparent_relocate(struct mem_cgroup *src, struct mem_cgroup *dst)
 {
 	int nid;
 
+	if (!lru_gen_enabled()) {
+		for_each_node(nid) {
+			enum lru_list lru;
+			struct lruvec *src_lruvec, *dst_lruvec;
+
+			src_lruvec = mem_cgroup_lruvec(src, NODE_DATA(nid));
+			dst_lruvec = mem_cgroup_lruvec(dst, NODE_DATA(nid));
+
+			dst_lruvec->anon_cost += src_lruvec->anon_cost;
+			dst_lruvec->file_cost += src_lruvec->file_cost;
+
+			for_each_lru(lru)
+				lruvec_reparent_lru(src_lruvec, dst_lruvec, lru);
+		}
+		return;
+	}
+
+#ifdef CONFIG_LRU_GEN
 	for_each_node(nid) {
 		enum lru_list lru;
 		int gen, type, zone, zid;
@@ -287,45 +324,8 @@ static void lruvec_reparent_relocate(struct mem_cgroup *src, struct mem_cgroup *
 			}
 		}
 	}
-}
-#else
-static void lruvec_reparent_lru(struct lruvec *src, struct lruvec *dst,
-                               enum lru_list lru)
-{
-       int zid;
-       struct mem_cgroup_per_node *mz_src, *mz_dst;
-
-       mz_src = container_of(src, struct mem_cgroup_per_node, lruvec);
-       mz_dst = container_of(dst, struct mem_cgroup_per_node, lruvec);
-
-       if (lru != LRU_UNEVICTABLE)
-               list_splice_tail_init(&src->lists[lru], &dst->lists[lru]);
-
-       for (zid = 0; zid < MAX_NR_ZONES; zid++) {
-               mz_dst->lru_zone_size[zid][lru] += mz_src->lru_zone_size[zid][lru];
-               mz_src->lru_zone_size[zid][lru] = 0;
-       }
-}
-
-static void lruvec_reparent_relocate(struct mem_cgroup *src, struct mem_cgroup *dst)
-{
-       int nid;
-
-       for_each_node(nid) {
-               enum lru_list lru;
-               struct lruvec *src_lruvec, *dst_lruvec;
-
-               src_lruvec = mem_cgroup_lruvec(src, NODE_DATA(nid));
-               dst_lruvec = mem_cgroup_lruvec(dst, NODE_DATA(nid));
-
-               dst_lruvec->anon_cost += src_lruvec->anon_cost;
-               dst_lruvec->file_cost += src_lruvec->file_cost;
-
-               for_each_lru(lru)
-                       lruvec_reparent_lru(src_lruvec, dst_lruvec, lru);
-       }
-}
 #endif
+}
 
 static void lruvec_reparent_unlock(struct mem_cgroup *src, struct mem_cgroup *dst)
 {