[PATCH 2/3] bcachefs: Fix maximum link count check when creating hard links

[Youling Tang <youling.tang@linux.dev>]

From: Youling Tang <tangyouling@kylinos.cn>

When I changed the maximum link count U32_MAX to 4 for testing purposes, I
discovered that for regular files, the maximum number of hard links created
could actually reach 5 (`inode->i_nlink`).

This occurs because `bi->bi_nlink` does not represent the actual `inode->i_nlink`
value, but rather equals `inode->i_nlink - nlink_bias(bi->bi_mode)`. Therefore,
the `bi->bi_nlink` check in bch2_inode_nlink_inc() needs to be corrected.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
---
NOTE: If pathconf is to be added to support _PC_LINK_MAX for bcachefs in
libc later, BCH_LINK_MAX can be defined as ((1U << 31) - 1U) like xfs.

 fs/bcachefs/bcachefs.h | 1 +
 fs/bcachefs/inode.c    | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/bcachefs.h b/fs/bcachefs/bcachefs.h
index ddfacad0f70c..9d5e6866b1b6 100644
--- a/fs/bcachefs/bcachefs.h
+++ b/fs/bcachefs/bcachefs.h
@@ -714,6 +714,7 @@ struct btree_debug {
 	unsigned		id;
 };
 
+#define BCH_LINK_MAX	U32_MAX
 #define BCH_TRANSACTIONS_NR 128
 
 struct btree_transaction_stats {
diff --git a/fs/bcachefs/inode.c b/fs/bcachefs/inode.c
index 5765144b4d65..eedffb505517 100644
--- a/fs/bcachefs/inode.c
+++ b/fs/bcachefs/inode.c
@@ -1192,7 +1192,7 @@ int bch2_inode_nlink_inc(struct bch_inode_unpacked *bi)
 	if (bi->bi_flags & BCH_INODE_unlinked)
 		bi->bi_flags &= ~BCH_INODE_unlinked;
 	else {
-		if (bi->bi_nlink == U32_MAX)
+		if (bi->bi_nlink == BCH_LINK_MAX - nlink_bias(bi->bi_mode))
 			return -BCH_ERR_too_many_links;
 
 		bi->bi_nlink++;
-- 
2.43.0