ext4: Fallback to vmalloc if kmalloc can't allocate s_flex_groups array

For very large filesystems, the s_flex_groups array can get quite big.
For example, a filesystem that can be resized up to 16TB will have
8192 flex groups (assuming the default flex_bg size of 16), so the
array is 96k, which is *very* marginal for kmalloc().  On the other
hand, a 160GB filesystem without the resize_inode feature will only
require 960 bytes.  So we try to allocate the array first using
kmalloc(), and if that fails, we'll try to use vmalloc() instead.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -20,6 +20,7 @@
 #include <linux/string.h>
 #include <linux/fs.h>
 #include <linux/time.h>
+#include <linux/vmalloc.h>
 #include <linux/jbd2.h>
 #include <linux/slab.h>
 #include <linux/init.h>
@@ -586,7 +587,10 @@ static void ext4_put_super(struct super_block *sb)
 	for (i = 0; i < sbi->s_gdb_count; i++)
 		brelse(sbi->s_group_desc[i]);
 	kfree(sbi->s_group_desc);
-	kfree(sbi->s_flex_groups);
+	if (is_vmalloc_addr(sbi->s_flex_groups))
+		vfree(sbi->s_flex_groups);
+	else
+		kfree(sbi->s_flex_groups);
 	percpu_counter_destroy(&sbi->s_freeblocks_counter);
 	percpu_counter_destroy(&sbi->s_freeinodes_counter);
 	percpu_counter_destroy(&sbi->s_dirs_counter);
@@ -1620,6 +1624,7 @@ static int ext4_fill_flex_info(struct super_block *sb)
 	ext4_group_t flex_group_count;
 	ext4_group_t flex_group;
 	int groups_per_flex = 0;
+	size_t size;
 	int i;
 
 	if (!sbi->s_es->s_log_groups_per_flex) {
@@ -1634,8 +1639,13 @@ static int ext4_fill_flex_info(struct super_block *sb)
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
 			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<
 			      EXT4_DESC_PER_BLOCK_BITS(sb))) / groups_per_flex;
-	sbi->s_flex_groups = kzalloc(flex_group_count *
-				     sizeof(struct flex_groups), GFP_KERNEL);
+	size = flex_group_count * sizeof(struct flex_groups);
+	sbi->s_flex_groups = kzalloc(size, GFP_KERNEL);
+	if (sbi->s_flex_groups == NULL) {
+		sbi->s_flex_groups = vmalloc(size);
+		if (sbi->s_flex_groups)
+			memset(sbi->s_flex_groups, 0, size);
+	}
 	if (sbi->s_flex_groups == NULL) {
 		printk(KERN_ERR "EXT4-fs: not enough memory for "
 				"%u flex groups\n", flex_group_count);
@@ -2842,6 +2852,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		sbi->s_journal = NULL;
 	}
 failed_mount3:
+	if (sbi->s_flex_groups) {
+		if (is_vmalloc_addr(sbi->s_flex_groups))
+			vfree(sbi->s_flex_groups);
+		else
+			kfree(sbi->s_flex_groups);
+	}
 	percpu_counter_destroy(&sbi->s_freeblocks_counter);
 	percpu_counter_destroy(&sbi->s_freeinodes_counter);
 	percpu_counter_destroy(&sbi->s_dirs_counter);