Re: [RFC PATCH] mm: reduce noise in show_mem for lowmem allocations

[Michal Hocko]

[Cc Dan]
Dan has brought up[1] that the use of gfp mask has confused his static
analyzer which assumes that GFP_HIGHUSER_MOVABLE implies a sleeping
allocation and that wouldn't be a great idea from the panic path. I
would add that most callers of this function would be really bad to
allocate.

The report itself is a false positive but it made me think a bit about
this. Even if the check is too simplistic I guess it resembles how many
developers are thinking (including me). If I see GFP_HIGHUSER_MOVABLE or
GF_KERNEL I automatically assume a sleeping allocation down the road.
And who know somebody might add one in the future even into show_mem
because the gfp parameter would be too tempting to not (ab)use.

My original intention was to use a natural allocation speak but this can
backfire so maybe it would be better to give the argument its real
meaning and that is the high_zone_idx. This is cryptic for code outside
of MM but that is not all that many callers and we can hide this fact
from them. In other words does the thing below looks better (incremental
for illustration, I will make it a proper patch if yes)?

[1] https://lore.kernel.org/all/Yw2ugrlZ8bwE5/hh@kili/?q=http%3A%2F%2Flkml.kernel.org%2Fr%2FYw2ugrlZ8bwE5%2Fhh%40kili

--- 
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 19b1a94b0c00..3d9782ea3fa7 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -1086,7 +1086,7 @@ cmds(struct pt_regs *excp)
 				memzcan();
 				break;
 			case 'i':
-				show_mem(0, NULL, GFP_HIGHUSER_MOVABLE);
+				show_mem(0, NULL);
 				break;
 			default:
 				termch = cmd;
diff --git a/arch/sparc/kernel/setup_32.c b/arch/sparc/kernel/setup_32.c
index 2a1e5641a7cc..c8e0dd99f370 100644
--- a/arch/sparc/kernel/setup_32.c
+++ b/arch/sparc/kernel/setup_32.c
@@ -83,7 +83,7 @@ static void prom_sync_me(void)
 			     "nop\n\t" : : "r" (&trapbase));
 
 	prom_printf("PROM SYNC COMMAND...\n");
-	show_free_areas(0, NULL, GFP_HIGHUSER_MOVABLE);
+	show_free_areas(0, NULL);
 	if (!is_idle_task(current)) {
 		local_irq_enable();
 		ksys_sync();
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index c6b2b42d2367..d2b2720db6ca 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -342,7 +342,7 @@ static const struct sysrq_key_op sysrq_ftrace_dump_op = {
 
 static void sysrq_handle_showmem(int key)
 {
-	show_mem(0, NULL, GFP_HIGHUSER_MOVABLE);
+	show_mem(0, NULL);
 }
 static const struct sysrq_key_op sysrq_showmem_op = {
 	.handler	= sysrq_handle_showmem,
diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 9ceeea531cf6..be8313cdbac3 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -606,7 +606,7 @@ static void fn_scroll_back(struct vc_data *vc)
 
 static void fn_show_mem(struct vc_data *vc)
 {
-	show_mem(0, NULL, GFP_HIGHUSER_MOVABLE);
+	show_mem(0, NULL);
 }
 
 static void fn_show_state(struct vc_data *vc)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e0d0ac52770c..0756798feaff 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1827,7 +1827,11 @@ extern void pagefault_out_of_memory(void);
  */
 #define SHOW_MEM_FILTER_NODES		(0x0001u)	/* disallowed nodes */
 
-extern void show_free_areas(unsigned int flags, nodemask_t *nodemask, gfp_t gfp_mask);
+extern void __show_free_areas(unsigned int flags, nodemask_t *nodemask, int max_zone_idx);
+static void __maybe_unused show_free_areas(unsigned int flags, nodemask_t *nodemask)
+{
+	__show_free_areas(flags, nodemask, MAX_NR_ZONES - 1);
+}
 
 #ifdef CONFIG_MMU
 extern bool can_do_mlock(void);
@@ -2563,7 +2567,12 @@ extern void calculate_min_free_kbytes(void);
 extern int __meminit init_per_zone_wmark_min(void);
 extern void mem_init(void);
 extern void __init mmap_init(void);
-extern void show_mem(unsigned int flags, nodemask_t *nodemask, gfp_t gfp_mask);
+
+extern void __show_mem(unsigned int flags, nodemask_t *nodemask, int max_zone_idx);
+static inline void show_mem(unsigned int flags, nodemask_t *nodemask)
+{
+	__show_mem(flags, nodemask, MAX_NR_ZONES - 1);
+}
 extern long si_mem_available(void);
 extern void si_meminfo(struct sysinfo * val);
 extern void si_meminfo_node(struct sysinfo *val, int nid);
diff --git a/init/initramfs.c b/init/initramfs.c
index ec966cb78363..18229cfe8906 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -63,7 +63,7 @@ static void panic_show_mem(const char *fmt, ...)
 {
 	va_list args;
 
-	show_mem(0, NULL, GFP_HIGHUSER_MOVABLE);
+	show_mem(0, NULL);
 	va_start(args, fmt);
 	panic(fmt, args);
 	va_end(args);
diff --git a/kernel/panic.c b/kernel/panic.c
index ba5f7691d15c..a3308af28a21 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -187,7 +187,7 @@ static void panic_print_sys_info(bool console_flush)
 		show_state();
 
 	if (panic_print & PANIC_PRINT_MEM_INFO)
-		show_mem(0, NULL, GFP_HIGHUSER_MOVABLE);
+		show_mem(0, NULL);
 
 	if (panic_print & PANIC_PRINT_TIMER_INFO)
 		sysrq_timer_list_show();
diff --git a/lib/show_mem.c b/lib/show_mem.c
index b97461a6c4bb..ade71096e241 100644
--- a/lib/show_mem.c
+++ b/lib/show_mem.c
@@ -8,13 +8,13 @@
 #include <linux/mm.h>
 #include <linux/cma.h>
 
-void show_mem(unsigned int filter, nodemask_t *nodemask, gfp_t gfp_mask)
+void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
 {
 	pg_data_t *pgdat;
 	unsigned long total = 0, reserved = 0, highmem = 0;
 
 	printk("Mem-Info:\n");
-	show_free_areas(filter, nodemask, gfp_mask);
+	show_free_areas(filter, nodemask, max_zone_idx);
 
 	for_each_online_pgdat(pgdat) {
 		int zoneid;
diff --git a/mm/nommu.c b/mm/nommu.c
index 46cff4a51fa0..9d7afc2d959e 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1047,7 +1047,7 @@ static int do_mmap_private(struct vm_area_struct *vma,
 enomem:
 	pr_err("Allocation of length %lu from process %d (%s) failed\n",
 	       len, current->pid, current->comm);
-	show_free_areas(0, NULL, GFP_KERNEL);
+	show_free_areas(0, NULL);
 	return -ENOMEM;
 }
 
@@ -1270,13 +1270,13 @@ unsigned long do_mmap(struct file *file,
 	kmem_cache_free(vm_region_jar, region);
 	pr_warn("Allocation of vma for %lu byte allocation from process %d failed\n",
 			len, current->pid);
-	show_free_areas(0, NULL, GFP_KERNEL);
+	show_free_areas(0, NULL);
 	return -ENOMEM;
 
 error_getting_region:
 	pr_warn("Allocation of vm region for %lu byte allocation from process %d failed\n",
 			len, current->pid);
-	show_free_areas(0, NULL, GFP_KERNEL);
+	show_free_areas(0, NULL);
 	return -ENOMEM;
 }
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ffeaad3b348d..94804504be9c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -461,7 +461,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
 	if (is_memcg_oom(oc))
 		mem_cgroup_print_oom_meminfo(oc->memcg);
 	else {
-		show_mem(SHOW_MEM_FILTER_NODES, oc->nodemask, oc->gfp_mask);
+		__show_mem(SHOW_MEM_FILTER_NODES, oc->nodemask, gfp_zone(oc->gfp_mask));
 		if (should_dump_unreclaim_slab())
 			dump_unreclaimable_slab();
 	}
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 208e3f8b38f8..110a16ea848a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4248,7 +4248,7 @@ static void warn_alloc_show_mem(gfp_t gfp_mask, nodemask_t *nodemask)
 	if (!in_task() || !(gfp_mask & __GFP_DIRECT_RECLAIM))
 		filter &= ~SHOW_MEM_FILTER_NODES;
 
-	show_mem(filter, nodemask, gfp_mask);
+	__show_mem(filter, nodemask, gfp_zone(gfp_mask));
 }
 
 void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
@@ -5944,10 +5944,9 @@ static bool node_has_managed_zones(pg_data_t *pgdat, int max_zone_idx)
  * SHOW_MEM_FILTER_NODES: suppress nodes that are not allowed by current's
  *   cpuset.
  */
-void show_free_areas(unsigned int filter, nodemask_t *nodemask, gfp_t gfp_mask)
+void __show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
 {
 	unsigned long free_pcp = 0;
-	int max_zone_idx = gfp_zone(gfp_mask);
 	int cpu;
 	struct zone *zone;
 	pg_data_t *pgdat;
-- 
Michal Hocko
SUSE Labs