mm, show_mem: suppress page counts in non-blockable contexts

On large systems with a lot of memory, walking all RAM to determine page
types may take a half second or even more.

In non-blockable contexts, the page allocator will emit a page allocation
failure warning unless __GFP_NOWARN is specified. In such contexts, irqs
are typically disabled and such a lengthy delay may even result in NMI
watchdog timeouts.

To fix this, suppress the page walk in such contexts when printing the
page allocation failure warning.

Signed-off-by: David Rientjes <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/arm/mm/init.c

···
 	printk("Mem-info:\n");
 	show_free_areas(filter);
 
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
+
 	for_each_bank (i, mi) {
 		struct membank *bank = &mi->bank[i];
 		unsigned int pfn1, pfn2;

arch/ia64/mm/contig.c

···
 	printk(KERN_INFO "Mem-info:\n");
 	show_free_areas(filter);
 	printk(KERN_INFO "Node memory in pages:\n");
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
 	for_each_online_pgdat(pgdat) {
 		unsigned long present;
 		unsigned long flags;

arch/ia64/mm/discontig.c

···
 
 	printk(KERN_INFO "Mem-info:\n");
 	show_free_areas(filter);
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
 	printk(KERN_INFO "Node memory in pages:\n");
 	for_each_online_pgdat(pgdat) {
 		unsigned long present;

arch/parisc/mm/init.c

···
 
 	printk(KERN_INFO "Mem-info:\n");
 	show_free_areas(filter);
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
 #ifndef CONFIG_DISCONTIGMEM
 	i = max_mapnr;
 	while (i-- > 0) {

arch/unicore32/mm/init.c

···
 	printk(KERN_DEFAULT "Mem-info:\n");
 	show_free_areas(filter);
 
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
+
 	for_each_bank(i, mi) {
 		struct membank *bank = &mi->bank[i];
 		unsigned int pfn1, pfn2;

include/linux/mm.h

···
  * Flags passed to show_mem() and show_free_areas() to suppress output in
  * various contexts.
  */
-#define SHOW_MEM_FILTER_NODES	(0x0001u)	/* filter disallowed nodes */
+#define SHOW_MEM_FILTER_NODES		(0x0001u)	/* disallowed nodes */
+#define SHOW_MEM_FILTER_PAGE_COUNT	(0x0002u)	/* page type count */
 
 extern void show_free_areas(unsigned int flags);
 extern bool skip_free_areas_node(unsigned int flags, int nid);

lib/show_mem.c

···
 	printk("Mem-Info:\n");
 	show_free_areas(filter);
 
+	if (filter & SHOW_MEM_FILTER_PAGE_COUNT)
+		return;
+
 	for_each_online_pgdat(pgdat) {
 		unsigned long i, flags;
 

mm/page_alloc.c

···
 		return;
 
 	/*
+	 * Walking all memory to count page types is very expensive and should
+	 * be inhibited in non-blockable contexts.
+	 */
+	if (!(gfp_mask & __GFP_WAIT))
+		filter |= SHOW_MEM_FILTER_PAGE_COUNT;
+
+	/*
 	 * This documents exceptions given to allocations in certain
 	 * contexts that are allowed to allocate outside current's set
 	 * of allowed nodes.