ext4: fix reference counting bug on block allocation error

commit 554a5ccc upstream.

If we hit this error when mounted with errors=continue or
errors=remount-ro:

    EXT4-fs error (device loop0): ext4_mb_mark_diskspace_used:2940: comm ext4.exe: Allocating blocks 5090-6081 which overlap fs metadata

then ext4_mb_new_blocks() will call ext4_mb_release_context() and try to
continue. However, ext4_mb_release_context() is the wrong thing to call
here since we are still actually using the allocation context.

Instead, just error out. We could retry the allocation, but there is a
possibility of getting stuck in an infinite loop instead, so this seems
safer.

[ Fixed up so we don't return EAGAIN to userspace. --tytso ]

Fixes: 8556e8f3 ("ext4: Don't allow new groups to be added during block allocation")
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[bwh: Backported to 3.16: use EIO instead of EFSCORRUPTED]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ext4/mballoc.c

@@ -2911,7 +2911,7 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
 		ext4_error(sb, "Allocating blocks %llu-%llu which overlap "
 			   "fs metadata", block, block+len);
 		/* File system mounted not to panic on error
-		 * Fix the bitmap and repeat the block allocation
+		 * Fix the bitmap and return EFSCORRUPTED
 		 * We leak some of the blocks here.
 		 */
 		ext4_lock_group(sb, ac->ac_b_ex.fe_group);
@@ -2920,7 +2920,7 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
 		ext4_unlock_group(sb, ac->ac_b_ex.fe_group);
 		err = ext4_handle_dirty_metadata(handle, NULL, bitmap_bh);
 		if (!err)
-			err = -EAGAIN;
+			err = -EIO;
 		goto out_err;
 	}
 
@@ -4489,18 +4489,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
 	}
 	if (likely(ac->ac_status == AC_STATUS_FOUND)) {
 		*errp = ext4_mb_mark_diskspace_used(ac, handle, reserv_clstrs);
-		if (*errp == -EAGAIN) {
-			/*
-			 * drop the reference that we took
-			 * in ext4_mb_use_best_found
-			 */
-			ext4_mb_release_context(ac);
-			ac->ac_b_ex.fe_group = 0;
-			ac->ac_b_ex.fe_start = 0;
-			ac->ac_b_ex.fe_len = 0;
-			ac->ac_status = AC_STATUS_CONTINUE;
-			goto repeat;
-		} else if (*errp) {
+		if (*errp) {
 			ext4_discard_allocated_blocks(ac);
 			goto errout;
 		} else {