[PATCH v3 5/6] btrfs: handle ENOMEM from btrfs_insert_dir_item() without aborting

[Jeff Layton <jlayton@kernel.org>]

Now that btrfs_insert_dir_item() returns -ENOMEM before modifying the
btree (thanks to delayed dir index pre-allocation), callers can handle
ENOMEM gracefully instead of aborting the transaction.

- btrfs_add_link(): add -ENOMEM to the recoverable errors alongside
  -EEXIST and -EOVERFLOW.
- btrfs_create_new_inode(): on -ENOMEM from btrfs_add_link(), orphan the
  newly-created inode instead of aborting. The inode item was already
  written with nlink 1, and discard_new_inode() marks it bad so eviction
  won't delete it. So clear_nlink() alone is not enough: persist nlink 0
  via btrfs_update_inode(), otherwise orphan cleanup would see nlink > 0,
  drop the orphan item, and leak the inode. Fall back to aborting only if
  that update also fails.

This turns a filesystem-killing abort into a graceful -ENOMEM return for
create(), mkdir(), mknod(), symlink(), and link() under memory pressure.

Assisted-by: LLM
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/btrfs/inode.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 3a2dca093c7d..5b79910d72f5 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6863,7 +6863,27 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 	} else {
 		ret = btrfs_add_link(trans, BTRFS_I(dir), BTRFS_I(inode), name,
 				     false, BTRFS_I(inode)->dir_index);
-		if (unlikely(ret)) {
+		if (ret == -ENOMEM) {
+			/*
+			 * Orphan the new inode instead of aborting. The inode
+			 * item was already written with nlink 1, and discard's
+			 * eviction won't delete a bad inode, so nlink 0 must be
+			 * persisted here or orphan cleanup would see nlink > 0,
+			 * drop the orphan item, and leak the inode.
+			 */
+			clear_nlink(inode);
+			/* btrfs_orphan_add() aborts the transaction on failure. */
+			ret = btrfs_orphan_add(trans, BTRFS_I(inode));
+			if (ret)
+				goto discard;
+			ret = btrfs_update_inode(trans, BTRFS_I(inode));
+			if (ret) {
+				btrfs_abort_transaction(trans, ret);
+				goto discard;
+			}
+			ret = -ENOMEM;
+			goto discard;
+		} else if (unlikely(ret)) {
 			btrfs_abort_transaction(trans, ret);
 			goto discard;
 		}
@@ -6925,7 +6945,7 @@ int btrfs_add_link(struct btrfs_trans_handle *trans,
 
 	ret = btrfs_insert_dir_item(trans, name, parent_inode, &key,
 				    btrfs_inode_type(inode), index, NULL);
-	if (ret == -EEXIST || ret == -EOVERFLOW)
+	if (ret == -EEXIST || ret == -EOVERFLOW || ret == -ENOMEM)
 		goto fail_dir_item;
 	else if (unlikely(ret)) {
 		btrfs_abort_transaction(trans, ret);

-- 
2.55.0