erofs: support dot-omitted directories

There's no need to record "." dirents in the directory data (while
they could be used for sanity checks, they aren't very useful.)
Omitting "." dirents also improves directory data deduplication.

Use a per-inode (instead of per-sb) flag to indicate if the "." dirent
is omitted or not, ensuring compatibility with incremental builds.  It
also reuses EROFS_I_NLINK_1_BIT, as it has very limited use cases for
directories with `nlink = 1`.

Emit the "." entry as the last virtual dirent in the directory because
it is _much_ less frequently used than the ".." dirent.  It also keeps
`f_pos` meaningful, as it strictly follows the directory data when it's
less than i_size.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250310095459.2620647-6-hsiangkao@linux.alibaba.com

fs/erofs/dir.c

@@ -90,6 +90,11 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 		ofs = 0;
 	}
 	erofs_put_metabuf(&buf);
+	if (EROFS_I(dir)->dot_omitted && ctx->pos == dir->i_size) {
+		if (!dir_emit_dot(f, ctx))
+			return 0;
+		++ctx->pos;
+	}
 	return err < 0 ? err : 0;
 }
 

fs/erofs/erofs_fs.h

@@ -116,6 +116,7 @@ static inline bool erofs_inode_is_data_compressed(unsigned int datamode)
 #define EROFS_I_VERSION_BIT	0
 #define EROFS_I_DATALAYOUT_BIT	1
 #define EROFS_I_NLINK_1_BIT	4	/* non-directory compact inodes only */
+#define EROFS_I_DOT_OMITTED_BIT	4	/* (directories) omit the `.` dirent */
 #define EROFS_I_ALL		((1 << (EROFS_I_NLINK_1_BIT + 1)) - 1)
 
 /* indicate chunk blkbits, thus 'chunksize = blocksize << chunk blkbits' */

fs/erofs/inode.c

@@ -136,8 +136,10 @@ static int erofs_read_inode(struct inode *inode)
 		goto err_out;
 	}
 	switch (inode->i_mode & S_IFMT) {
-	case S_IFREG:
 	case S_IFDIR:
+		vi->dot_omitted = (ifmt >> EROFS_I_DOT_OMITTED_BIT) & 1;
+		fallthrough;
+	case S_IFREG:
 	case S_IFLNK:
 		vi->startblk = le32_to_cpu(copied.i_u.startblk_lo) |
 			((u64)le16_to_cpu(copied.i_nb.startblk_hi) << 32);

fs/erofs/internal.h

@@ -245,6 +245,7 @@ struct erofs_inode {
 
 	unsigned char datalayout;
 	unsigned char inode_isize;
+	bool dot_omitted;
 	unsigned int xattr_isize;
 
 	unsigned int xattr_name_filter;