UBIFS: handle allocation failures in UBIFS write path

Running kernel 2.6.37, my PPC-based device occasionally gets an
order-2 allocation failure in UBIFS, which causes the root FS to
become unwritable:

kswapd0: page allocation failure. order:2, mode:0x4050
Call Trace:
[c787dc30] [c00085b8] show_stack+0x7c/0x194 (unreliable)
[c787dc70] [c0061aec] __alloc_pages_nodemask+0x4f0/0x57c
[c787dd00] [c0061b98] __get_free_pages+0x20/0x50
[c787dd10] [c00e4f88] ubifs_jnl_write_data+0x54/0x200
[c787dd50] [c00e82d4] do_writepage+0x94/0x198
[c787dd90] [c00675e4] shrink_page_list+0x40c/0x77c
[c787de40] [c0067de0] shrink_inactive_list+0x1e0/0x370
[c787de90] [c0068224] shrink_zone+0x2b4/0x2b8
[c787df00] [c0068854] kswapd+0x408/0x5d4
[c787dfb0] [c0037bcc] kthread+0x80/0x84
[c787dff0] [c000ef44] kernel_thread+0x4c/0x68

Similar problems were encountered last April by Tomasz Stanislawski:

http://patchwork.ozlabs.org/patch/50965/

This patch implements Artem's suggested fix: fall back to a
mutex-protected static buffer, allocated at mount time.  I tested it
by forcing execution down the failure path, and didn't see any ill
effects.

Artem: massaged the patch a little, improved it so that we'd not
allocate the write reserve buffer when we are in R/O mode.
Signed-off-by: Matthew L. Creech <mlcreech@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/journal.c

@@ -690,7 +690,7 @@ int ubifs_jnl_write_data(struct ubifs_info *c, const struct inode *inode,
 {
 	struct ubifs_data_node *data;
 	int err, lnum, offs, compr_type, out_len;
-	int dlen = UBIFS_DATA_NODE_SZ + UBIFS_BLOCK_SIZE * WORST_COMPR_FACTOR;
+	int dlen = COMPRESSED_DATA_NODE_BUF_SZ, allocated = 1;
 	struct ubifs_inode *ui = ubifs_inode(inode);
 
 	dbg_jnl("ino %lu, blk %u, len %d, key %s",
@@ -698,9 +698,19 @@ int ubifs_jnl_write_data(struct ubifs_info *c, const struct inode *inode,
 		DBGKEY(key));
 	ubifs_assert(len <= UBIFS_BLOCK_SIZE);
 
-	data = kmalloc(dlen, GFP_NOFS);
-	if (!data)
-		return -ENOMEM;
+	data = kmalloc(dlen, GFP_NOFS | __GFP_NOWARN);
+	if (!data) {
+		/*
+		 * Fall-back to the write reserve buffer. Note, we might be
+		 * currently on the memory reclaim path, when the kernel is
+		 * trying to free some memory by writing out dirty pages. The
+		 * write reserve buffer helps us to guarantee that we are
+		 * always able to write the data.
+		 */
+		allocated = 0;
+		mutex_lock(&c->write_reserve_mutex);
+		data = c->write_reserve_buf;
+	}
 
 	data->ch.node_type = UBIFS_DATA_NODE;
 	key_write(c, key, &data->key);
@@ -736,6 +746,9 @@ int ubifs_jnl_write_data(struct ubifs_info *c, const struct inode *inode,
 		goto out_ro;
 
 	finish_reservation(c);
+	if (!allocated)
+		mutex_unlock(&c->write_reserve_mutex);
+	else
 		kfree(data);
 	return 0;
 
@@ -745,6 +758,9 @@ int ubifs_jnl_write_data(struct ubifs_info *c, const struct inode *inode,
 	ubifs_ro_mode(c, err);
 	finish_reservation(c);
 out_free:
+	if (!allocated)
+		mutex_unlock(&c->write_reserve_mutex);
+	else
 		kfree(data);
 	return err;
 }

fs/ubifs/super.c

@@ -1213,6 +1213,13 @@ static int mount_ubifs(struct ubifs_info *c)
 	if (c->bulk_read == 1)
 		bu_init(c);
 
+	if (!c->ro_mount) {
+		c->write_reserve_buf = kmalloc(COMPRESSED_DATA_NODE_BUF_SZ,
+					       GFP_KERNEL);
+		if (!c->write_reserve_buf)
+			goto out_free;
+	}
+
 	c->mounting = 1;
 
 	err = ubifs_read_superblock(c);
@@ -1482,6 +1489,7 @@ static int mount_ubifs(struct ubifs_info *c)
 out_cbuf:
 	kfree(c->cbuf);
 out_free:
+	kfree(c->write_reserve_buf);
 	kfree(c->bu.buf);
 	vfree(c->ileb_buf);
 	vfree(c->sbuf);
@@ -1520,6 +1528,7 @@ static void ubifs_umount(struct ubifs_info *c)
 	kfree(c->cbuf);
 	kfree(c->rcvrd_mst_node);
 	kfree(c->mst_node);
+	kfree(c->write_reserve_buf);
 	kfree(c->bu.buf);
 	vfree(c->ileb_buf);
 	vfree(c->sbuf);
@@ -1605,6 +1614,10 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 		goto out;
 	}
 
+	c->write_reserve_buf = kmalloc(COMPRESSED_DATA_NODE_BUF_SZ, GFP_KERNEL);
+	if (!c->write_reserve_buf)
+		goto out;
+
 	err = ubifs_lpt_init(c, 0, 1);
 	if (err)
 		goto out;
@@ -1669,6 +1682,8 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 		c->bgt = NULL;
 	}
 	free_wbufs(c);
+	kfree(c->write_reserve_buf);
+	c->write_reserve_buf = NULL;
 	vfree(c->ileb_buf);
 	c->ileb_buf = NULL;
 	ubifs_lpt_free(c, 1);
@@ -1712,6 +1727,8 @@ static void ubifs_remount_ro(struct ubifs_info *c)
 	free_wbufs(c);
 	vfree(c->orph_buf);
 	c->orph_buf = NULL;
+	kfree(c->write_reserve_buf);
+	c->write_reserve_buf = NULL;
 	vfree(c->ileb_buf);
 	c->ileb_buf = NULL;
 	ubifs_lpt_free(c, 1);
@@ -1942,6 +1959,7 @@ static int ubifs_fill_super(struct super_block *sb, void *data, int silent)
 	mutex_init(&c->mst_mutex);
 	mutex_init(&c->umount_mutex);
 	mutex_init(&c->bu_mutex);
+	mutex_init(&c->write_reserve_mutex);
 	init_waitqueue_head(&c->cmt_wq);
 	c->buds = RB_ROOT;
 	c->old_idx = RB_ROOT;

fs/ubifs/ubifs.h

@@ -151,6 +151,12 @@
  */
 #define WORST_COMPR_FACTOR 2
 
+/*
+ * How much memory is needed for a buffer where we comress a data node.
+ */
+#define COMPRESSED_DATA_NODE_BUF_SZ \
+	(UBIFS_DATA_NODE_SZ + UBIFS_BLOCK_SIZE * WORST_COMPR_FACTOR)
+
 /* Maximum expected tree height for use by bottom_up_buf */
 #define BOTTOM_UP_HEIGHT 64
 
@@ -1005,6 +1011,11 @@ struct ubifs_debug_info;
  * @bu_mutex: protects the pre-allocated bulk-read buffer and @c->bu
  * @bu: pre-allocated bulk-read information
  *
+ * @write_reserve_mutex: protects @write_reserve_buf
+ * @write_reserve_buf: on the write path we allocate memory, which might
+ *                     sometimes be unavailable, in which case we use this
+ *                     write reserve buffer
+ *
  * @log_lebs: number of logical eraseblocks in the log
  * @log_bytes: log size in bytes
  * @log_last: last LEB of the log
@@ -1256,6 +1267,9 @@ struct ubifs_info {
 	struct mutex bu_mutex;
 	struct bu_info bu;
 
+	struct mutex write_reserve_mutex;
+	void *write_reserve_buf;
+
 	int log_lebs;
 	long long log_bytes;
 	int log_last;