erofs: introduce on-disk lz4 fs configurations

Introduce z_erofs_lz4_cfgs to store all lz4 configurations.
Currently it's only max_distance, but will be used for new
features later.

Link: https://lore.kernel.org/r/20210329012308.28743-4-hsiangkao@aol.comReviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/decompressor.c

@@ -29,9 +29,20 @@ struct z_erofs_decompressor {
 };
 
 int z_erofs_load_lz4_config(struct super_block *sb,
-			    struct erofs_super_block *dsb)
+			    struct erofs_super_block *dsb,
+			    struct z_erofs_lz4_cfgs *lz4, int size)
 {
-	u16 distance = le16_to_cpu(dsb->lz4_max_distance);
+	u16 distance;
+
+	if (lz4) {
+		if (size < sizeof(struct z_erofs_lz4_cfgs)) {
+			erofs_err(sb, "invalid lz4 cfgs, size=%u", size);
+			return -EINVAL;
+		}
+		distance = le16_to_cpu(lz4->max_distance);
+	} else {
+		distance = le16_to_cpu(dsb->lz4_max_distance);
+	}
 
 	EROFS_SB(sb)->lz4.max_distance_pages = distance ?
 					DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :

fs/erofs/erofs_fs.h

@@ -200,6 +200,12 @@ enum {
 	Z_EROFS_COMPRESSION_MAX
 };
 
+/* 14 bytes (+ length field = 16 bytes) */
+struct z_erofs_lz4_cfgs {
+	__le16 max_distance;
+	u8 reserved[12];
+} __packed;
+
 /*
  * bit 0 : COMPACTED_2B indexes (0 - off; 1 - on)
  *  e.g. for 4k logical cluster size,      4B        if compacted 2B is off;

fs/erofs/internal.h

@@ -439,7 +439,8 @@ int erofs_try_to_free_all_cached_pages(struct erofs_sb_info *sbi,
 int erofs_try_to_free_cached_page(struct address_space *mapping,
 				  struct page *page);
 int z_erofs_load_lz4_config(struct super_block *sb,
-			    struct erofs_super_block *dsb);
+			    struct erofs_super_block *dsb,
+			    struct z_erofs_lz4_cfgs *lz4, int len);
 #else
 static inline void erofs_shrinker_register(struct super_block *sb) {}
 static inline void erofs_shrinker_unregister(struct super_block *sb) {}
@@ -448,9 +449,10 @@ static inline void erofs_exit_shrinker(void) {}
 static inline int z_erofs_init_zip_subsystem(void) { return 0; }
 static inline void z_erofs_exit_zip_subsystem(void) {}
 static inline int z_erofs_load_lz4_config(struct super_block *sb,
-				struct erofs_super_block *dsb)
+				  struct erofs_super_block *dsb,
+				  struct z_erofs_lz4_cfgs *lz4, int len)
 {
-	if (dsb->lz4_max_distance) {
+	if (lz4 || dsb->lz4_max_distance) {
 		erofs_err(sb, "lz4 algorithm isn't enabled");
 		return -EINVAL;
 	}

fs/erofs/super.c

@@ -189,7 +189,7 @@ static int erofs_read_superblock(struct super_block *sb)
 	}
 
 	/* parse on-disk compression configurations */
-	ret = z_erofs_load_lz4_config(sb, dsb);
+	ret = z_erofs_load_lz4_config(sb, dsb, NULL, 0);
 out:
 	kunmap(page);
 	put_page(page);