erofs: fix long xattr name prefix placement

Currently, xattr name prefixes are forcibly placed into the packed
inode if the fragments feature is enabled, and users have no option
to put them in plain form directly on disk.

This is inflexible. First, as mentioned above, users should be able
to store unwrapped long xattr name prefixes unconditionally
(COMPAT_PLAIN_XATTR_PFX). Second, since we now have the new metabox
inode to store metadata, it should be used when available instead
of the packed inode.

Fixes: 414091322c63 ("erofs: implement metadata compression")
Signed-off-by: Gao Xiang <[e-mail]>

fs/erofs/erofs_fs.h

···
 /* to allow for x86 boot sectors and other oddities. */
 #define EROFS_SUPER_OFFSET      1024
 
-#define EROFS_FEATURE_COMPAT_SB_CHKSUM          0x00000001
-#define EROFS_FEATURE_COMPAT_MTIME              0x00000002
-#define EROFS_FEATURE_COMPAT_XATTR_FILTER	0x00000004
+#define EROFS_FEATURE_COMPAT_SB_CHKSUM			0x00000001
+#define EROFS_FEATURE_COMPAT_MTIME			0x00000002
+#define EROFS_FEATURE_COMPAT_XATTR_FILTER		0x00000004
 #define EROFS_FEATURE_COMPAT_SHARED_EA_IN_METABOX	0x00000008
+#define EROFS_FEATURE_COMPAT_PLAIN_XATTR_PFX		0x00000010
+
 
 /*
  * Any bits that aren't in EROFS_ALL_FEATURE_INCOMPAT should

fs/erofs/internal.h

···
 EROFS_FEATURE_FUNCS(sb_chksum, compat, COMPAT_SB_CHKSUM)
 EROFS_FEATURE_FUNCS(xattr_filter, compat, COMPAT_XATTR_FILTER)
 EROFS_FEATURE_FUNCS(shared_ea_in_metabox, compat, COMPAT_SHARED_EA_IN_METABOX)
+EROFS_FEATURE_FUNCS(plain_xattr_pfx, compat, COMPAT_PLAIN_XATTR_PFX)
 
 static inline u64 erofs_nid_to_ino64(struct erofs_sb_info *sbi, erofs_nid_t nid)
 {

fs/erofs/xattr.c

···
 	erofs_off_t pos = (erofs_off_t)sbi->xattr_prefix_start << 2;
 	struct erofs_xattr_prefix_item *pfs;
 	int ret = 0, i, len;
+	bool plain = erofs_sb_has_plain_xattr_pfx(sbi);
 
 	if (!sbi->xattr_prefix_count)
 		return 0;
···
 	if (!pfs)
 		return -ENOMEM;
 
-	if (sbi->packed_inode)
-		buf.mapping = sbi->packed_inode->i_mapping;
-	else
+	if (!plain) {
+		if (erofs_sb_has_metabox(sbi))
+			(void)erofs_init_metabuf(&buf, sb, true);
+		else if (sbi->packed_inode)
+			buf.mapping = sbi->packed_inode->i_mapping;
+		else
+			plain = true;
+	}
+	if (plain)
 		(void)erofs_init_metabuf(&buf, sb, false);
 
 	for (i = 0; i < sbi->xattr_prefix_count; i++) {