fs/seq_file: fallback to vmalloc allocation

There are a couple of seq_files which use the single_open() interface.
This interface requires that the whole output must fit into a single
buffer.

E.g. for /proc/stat allocation failures have been observed because an
order-4 memory allocation failed due to memory fragmentation. In such
situations reading /proc/stat is not possible anymore.

Therefore change the seq_file code to fallback to vmalloc allocations
which will usually result in a couple of order-0 allocations and hence
also work if memory is fragmented.

For reference a call trace where reading from /proc/stat failed:

sadc: page allocation failure: order:4, mode:0x1040d0
CPU: 1 PID: 192063 Comm: sadc Not tainted 3.10.0-123.el7.s390x #1
[...]
Call Trace:
show_stack+0x6c/0xe8
warn_alloc_failed+0xd6/0x138
__alloc_pages_nodemask+0x9da/0xb68
__get_free_pages+0x2e/0x58
kmalloc_order_trace+0x44/0xc0
stat_open+0x5a/0xd8
proc_reg_open+0x8a/0x140
do_dentry_open+0x1bc/0x2c8
finish_open+0x46/0x60
do_last+0x382/0x10d0
path_openat+0xc8/0x4f8
do_filp_open+0x46/0xa8
do_sys_open+0x114/0x1f0
sysc_tracego+0x14/0x1a

Signed-off-by: Heiko Carstens <[e-mail]>
Tested-by: David Rientjes <[e-mail]>
Cc: Ian Kent <[e-mail]>
Cc: Hendrik Brueckner <[e-mail]>
Cc: Thorsten Diehl <[e-mail]>
Cc: Andrea Righi <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Stefan Bader <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/seq_file.c

···
 #include <linux/fs.h>
 #include <linux/export.h>
 #include <linux/seq_file.h>
+#include <linux/vmalloc.h>
 #include <linux/slab.h>
 #include <linux/cred.h>
+#include <linux/mm.h>
 
 #include <asm/uaccess.h>
 #include <asm/page.h>
···
 static void seq_set_overflow(struct seq_file *m)
 {
 	m->count = m->size;
+}
+
+static void *seq_buf_alloc(unsigned long size)
+{
+	void *buf;
+
+	buf = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
+	if (!buf && size > PAGE_SIZE)
+		buf = vmalloc(size);
+	return buf;
 }
 
 /**
···
 		return 0;
 	}
 	if (!m->buf) {
-		m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
+		m->buf = seq_buf_alloc(m->size = PAGE_SIZE);
 		if (!m->buf)
 			return -ENOMEM;
 	}
···
 
 Eoverflow:
 	m->op->stop(m, p);
-	kfree(m->buf);
+	kvfree(m->buf);
 	m->count = 0;
-	m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
+	m->buf = seq_buf_alloc(m->size <<= 1);
 	return !m->buf ? -ENOMEM : -EAGAIN;
 }
 
···
 
 	/* grab buffer if we didn't have one */
 	if (!m->buf) {
-		m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
+		m->buf = seq_buf_alloc(m->size = PAGE_SIZE);
 		if (!m->buf)
 			goto Enomem;
 	}
···
 		if (m->count < m->size)
 			goto Fill;
 		m->op->stop(m, p);
-		kfree(m->buf);
+		kvfree(m->buf);
 		m->count = 0;
-		m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
+		m->buf = seq_buf_alloc(m->size <<= 1);
 		if (!m->buf)
 			goto Enomem;
 		m->version = 0;
···
 int seq_release(struct inode *inode, struct file *file)
 {
 	struct seq_file *m = file->private_data;
-	kfree(m->buf);
+	kvfree(m->buf);
 	kfree(m);
 	return 0;
 }
···
 int single_open_size(struct file *file, int (*show)(struct seq_file *, void *),
 		void *data, size_t size)
 {
-	char *buf = kmalloc(size, GFP_KERNEL);
+	char *buf = seq_buf_alloc(size);
 	int ret;
 	if (!buf)
 		return -ENOMEM;
 	ret = single_open(file, show, data);
 	if (ret) {
-		kfree(buf);
+		kvfree(buf);
 		return ret;
 	}
 	((struct seq_file *)file->private_data)->buf = buf;