Go read-only if we can't read ephemeral objects

Failing to read an ephemeral object does not abort the transaction, it
just returns an error. The problem is that, by this point, nx_eph_list
is already set: the next time a transaction starts the reads will get
skipped, and things will proceed as if there was no error.

I got this series of messages from xfstests (no idea which test, but
clearly one that fakes I/O errors):

  [18920.212351] Buffer I/O error on dev dm-0, logical block 3932144, async page read
  [18920.515341] APFS (dm-0:0): failed to read cpm block (apfs_read_single_cpm_block:231)
  [18920.520378] APFS (dm-0:0): failed to read cpm block 0 (apfs_read_ephemeral_objects:300)
  [18920.524371] APFS (dm-0:0): failed to read the ephemeral objects (apfs_transaction_start:384)
  [18920.529524] APFS (dm-0:0): no mapping for oid 0x400 (apfs_ephemeral_object_lookup:216)
  [18920.532784] APFS (dm-0:0): no spaceman object for oid 0x400 (apfs_read_spaceman:744)
  [18920.536222] APFS (dm-0:0): failed to read the spaceman (apfs_transaction_start:395)
  [18920.538634] APFS (dm-0:0): aborting transaction (apfs_transaction_abort:1001)
  [18920.646469] APFS (dm-0:0): no mapping for oid 0x400 (apfs_ephemeral_object_lookup:216)
  [18920.653922] APFS (dm-0:0): no spaceman object for oid 0x400 (apfs_spaceman_get_free_blkcnt:1468)
  [18920.766489] APFS (dm-0:0): no mapping for oid 0x400 (apfs_ephemeral_object_lookup:216)
  [18920.770446] APFS (dm-0:0): no spaceman object for oid 0x400 (apfs_spaceman_get_free_blkcnt:1468)

No harm was done because the next transaction still got aborted as soon
as we tried to read the spaceman. But, if at some point in the future we
stop fully flushing free queues on each start, we could run into more
subtle bugs involving missing fq nodes.

This all seems unlikely but, to be safe, make the mount read-only when
we fail to get the ephemeral objects. It's really the same situation as
any other abort, so it's not a problem.

Signed-off-by: Ernesto A. Fernández <[email protected]>

Author: eafer

transaction.c

@@ -263,6 +263,8 @@ static int apfs_read_single_cpm_block(struct super_block *sb, u64 cpm_bno)
 	return err;
 }
 
+static void apfs_force_readonly(struct apfs_nxsb_info *nxi);
+
 /**
  * apfs_read_ephemeral_objects - Read all ephemeral objects to memory
  * @sb:	superblock structure
@@ -298,6 +300,8 @@ int apfs_read_ephemeral_objects(struct super_block *sb)
 		err = apfs_read_single_cpm_block(sb, cpm_bno);
 		if (err) {
 			apfs_err(sb, "failed to read cpm block %u", i);
+			/* No transaction to abort yet */
+			apfs_force_readonly(nxi);
 			return err;
 		}
 	}