jbd2: do not fail journal because of frozen_buffer allocation failure

Journal transaction might fail prematurely because the frozen_buffer
is allocated by GFP_NOFS request:
[   72.440013] do_get_write_access: OOM for frozen_buffer
[   72.440014] EXT4-fs: ext4_reserve_inode_write:4729: aborting transaction: Out of memory in __ext4_journal_get_write_access
[   72.440015] EXT4-fs error (device sda1) in ext4_reserve_inode_write:4735: Out of memory
(...snipped....)
[   72.495559] do_get_write_access: OOM for frozen_buffer
[   72.495560] EXT4-fs: ext4_reserve_inode_write:4729: aborting transaction: Out of memory in __ext4_journal_get_write_access
[   72.496839] do_get_write_access: OOM for frozen_buffer
[   72.496841] EXT4-fs: ext4_reserve_inode_write:4729: aborting transaction: Out of memory in __ext4_journal_get_write_access
[   72.505766] Aborting journal on device sda1-8.
[   72.505851] EXT4-fs (sda1): Remounting filesystem read-only

This wasn't a problem until "mm: page_alloc: do not lock up GFP_NOFS
allocations upon OOM" because small GPF_NOFS allocations never failed.
This allocation seems essential for the journal and GFP_NOFS is too
restrictive to the memory allocator so let's use __GFP_NOFAIL here to
emulate the previous behavior.
Signed-off-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -966,14 +966,8 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 		if (!frozen_buffer) {
 			JBUFFER_TRACE(jh, "allocate memory for buffer");
 			jbd_unlock_bh_state(bh);
-			frozen_buffer = jbd2_alloc(jh2bh(jh)->b_size, GFP_NOFS);
-			if (!frozen_buffer) {
-				printk(KERN_ERR "%s: OOM for frozen_buffer\n",
-				       __func__);
-				JBUFFER_TRACE(jh, "oom!");
-				error = -ENOMEM;
-				goto out;
-			}
+			frozen_buffer = jbd2_alloc(jh2bh(jh)->b_size,
+						   GFP_NOFS | __GFP_NOFAIL);
 			goto repeat;
 		}
 		jh->b_frozen_data = frozen_buffer;
@@ -1226,15 +1220,9 @@ int jbd2_journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
 		goto out;
 
 repeat:
-	if (!jh->b_committed_data) {
-		committed_data = jbd2_alloc(jh2bh(jh)->b_size, GFP_NOFS);
-		if (!committed_data) {
-			printk(KERN_ERR "%s: No memory for committed data\n",
-				__func__);
-			err = -ENOMEM;
-			goto out;
-		}
-	}
+	if (!jh->b_committed_data)
+		committed_data = jbd2_alloc(jh2bh(jh)->b_size,
+					    GFP_NOFS|__GFP_NOFAIL);
 
 	jbd_lock_bh_state(bh);
 	if (!jh->b_committed_data) {