erofs: add on-disk compression configurations

Add a bitmap for available compression algorithms and a variable-sized
on-disk table for compression options in preparation for upcoming big
pcluster and LZMA algorithm, which follows the end of super block.

To parse the compression options, the bitmap is scanned one by one.
For each available algorithm, there is data followed by 2-byte `length'
correspondingly (it's enough for most cases, or entire fs blocks should
be used.)

With such available algorithm bitmap, kernel itself can also refuse to
mount such filesystem if any unsupported compression algorithm exists.

Note that COMPR_CFGS feature will be enabled with BIG_PCLUSTER.

Link: https://lore.kernel.org/r/20210329100012.12980-1-hsiangkao@aol.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/decompressor.c

@@ -41,7 +41,7 @@ int z_erofs_load_lz4_config(struct super_block *sb,
 		}
 		distance = le16_to_cpu(lz4->max_distance);
 	} else {
-		distance = le16_to_cpu(dsb->lz4_max_distance);
+		distance = le16_to_cpu(dsb->u1.lz4_max_distance);
 	}
 
 	EROFS_SB(sb)->lz4.max_distance_pages = distance ?

fs/erofs/erofs_fs.h

@@ -18,15 +18,18 @@
  * be incompatible with this kernel version.
  */
 #define EROFS_FEATURE_INCOMPAT_LZ4_0PADDING	0x00000001
+#define EROFS_FEATURE_INCOMPAT_COMPR_CFGS	0x00000002
 #define EROFS_ALL_FEATURE_INCOMPAT		EROFS_FEATURE_INCOMPAT_LZ4_0PADDING
 
-/* 128-byte erofs on-disk super block */
+#define EROFS_SB_EXTSLOT_SIZE	16
+
+/* erofs on-disk super block (currently 128 bytes) */
 struct erofs_super_block {
 	__le32 magic;           /* file system magic number */
 	__le32 checksum;        /* crc32c(super_block) */
 	__le32 feature_compat;
 	__u8 blkszbits;         /* support block_size == PAGE_SIZE only */
-	__u8 reserved;
+	__u8 sb_extslots;	/* superblock size = 128 + sb_extslots * 16 */
 
 	__le16 root_nid;	/* nid of root directory */
 	__le64 inos;            /* total valid ino # (== f_files - f_favail) */
@@ -39,8 +42,12 @@ struct erofs_super_block {
 	__u8 uuid[16];          /* 128-bit uuid for volume */
 	__u8 volume_name[16];   /* volume name */
 	__le32 feature_incompat;
-	/* customized lz4 sliding window size instead of 64k by default */
-	__le16 lz4_max_distance;
+	union {
+		/* bitmap for available compression algorithms */
+		__le16 available_compr_algs;
+		/* customized sliding window size instead of 64k by default */
+		__le16 lz4_max_distance;
+	} __packed u1;
 	__u8 reserved2[42];
 };
 
@@ -199,6 +206,7 @@ enum {
 	Z_EROFS_COMPRESSION_LZ4	= 0,
 	Z_EROFS_COMPRESSION_MAX
 };
+#define Z_EROFS_ALL_COMPR_ALGS		(1 << (Z_EROFS_COMPRESSION_MAX - 1))
 
 /* 14 bytes (+ length field = 16 bytes) */
 struct z_erofs_lz4_cfgs {

fs/erofs/internal.h

@@ -75,6 +75,7 @@ struct erofs_sb_info {
 	struct xarray managed_pslots;
 
 	unsigned int shrinker_run_no;
+	u16 available_compr_algs;
 
 	/* pseudo inode to manage cached pages */
 	struct inode *managed_cache;
@@ -90,6 +91,7 @@ struct erofs_sb_info {
 	/* inode slot unit size in bit shift */
 	unsigned char islotbits;
 
+	u32 sb_size;			/* total superblock size */
 	u32 build_time_nsec;
 	u64 build_time;
 
@@ -233,6 +235,7 @@ static inline bool erofs_sb_has_##name(struct erofs_sb_info *sbi) \
 }
 
 EROFS_FEATURE_FUNCS(lz4_0padding, incompat, INCOMPAT_LZ4_0PADDING)
+EROFS_FEATURE_FUNCS(compr_cfgs, incompat, INCOMPAT_COMPR_CFGS)
 EROFS_FEATURE_FUNCS(sb_chksum, compat, COMPAT_SB_CHKSUM)
 
 /* atomic flag definitions */
@@ -452,7 +455,7 @@ static inline int z_erofs_load_lz4_config(struct super_block *sb,
 				  struct erofs_super_block *dsb,
 				  struct z_erofs_lz4_cfgs *lz4, int len)
 {
-	if (lz4 || dsb->lz4_max_distance) {
+	if (lz4 || dsb->u1.lz4_max_distance) {
 		erofs_err(sb, "lz4 algorithm isn't enabled");
 		return -EINVAL;
 	}

fs/erofs/super.c

@@ -122,6 +122,136 @@ static bool check_layout_compatibility(struct super_block *sb,
 	return true;
 }
 
+#ifdef CONFIG_EROFS_FS_ZIP
+/* read variable-sized metadata, offset will be aligned by 4-byte */
+static void *erofs_read_metadata(struct super_block *sb, struct page **pagep,
+				 erofs_off_t *offset, int *lengthp)
+{
+	struct page *page = *pagep;
+	u8 *buffer, *ptr;
+	int len, i, cnt;
+	erofs_blk_t blk;
+
+	*offset = round_up(*offset, 4);
+	blk = erofs_blknr(*offset);
+
+	if (!page || page->index != blk) {
+		if (page) {
+			unlock_page(page);
+			put_page(page);
+		}
+		page = erofs_get_meta_page(sb, blk);
+		if (IS_ERR(page))
+			goto err_nullpage;
+	}
+
+	ptr = kmap(page);
+	len = le16_to_cpu(*(__le16 *)&ptr[erofs_blkoff(*offset)]);
+	if (!len)
+		len = U16_MAX + 1;
+	buffer = kmalloc(len, GFP_KERNEL);
+	if (!buffer) {
+		buffer = ERR_PTR(-ENOMEM);
+		goto out;
+	}
+	*offset += sizeof(__le16);
+	*lengthp = len;
+
+	for (i = 0; i < len; i += cnt) {
+		cnt = min(EROFS_BLKSIZ - (int)erofs_blkoff(*offset), len - i);
+		blk = erofs_blknr(*offset);
+
+		if (!page || page->index != blk) {
+			if (page) {
+				kunmap(page);
+				unlock_page(page);
+				put_page(page);
+			}
+			page = erofs_get_meta_page(sb, blk);
+			if (IS_ERR(page)) {
+				kfree(buffer);
+				goto err_nullpage;
+			}
+			ptr = kmap(page);
+		}
+		memcpy(buffer + i, ptr + erofs_blkoff(*offset), cnt);
+		*offset += cnt;
+	}
+out:
+	kunmap(page);
+	*pagep = page;
+	return buffer;
+err_nullpage:
+	*pagep = NULL;
+	return page;
+}
+
+static int erofs_load_compr_cfgs(struct super_block *sb,
+				 struct erofs_super_block *dsb)
+{
+	struct erofs_sb_info *sbi;
+	struct page *page;
+	unsigned int algs, alg;
+	erofs_off_t offset;
+	int size, ret;
+
+	sbi = EROFS_SB(sb);
+	sbi->available_compr_algs = le16_to_cpu(dsb->u1.available_compr_algs);
+
+	if (sbi->available_compr_algs & ~Z_EROFS_ALL_COMPR_ALGS) {
+		erofs_err(sb, "try to load compressed fs with unsupported algorithms %x",
+			  sbi->available_compr_algs & ~Z_EROFS_ALL_COMPR_ALGS);
+		return -EINVAL;
+	}
+
+	offset = EROFS_SUPER_OFFSET + sbi->sb_size;
+	page = NULL;
+	alg = 0;
+	ret = 0;
+
+	for (algs = sbi->available_compr_algs; algs; algs >>= 1, ++alg) {
+		void *data;
+
+		if (!(algs & 1))
+			continue;
+
+		data = erofs_read_metadata(sb, &page, &offset, &size);
+		if (IS_ERR(data)) {
+			ret = PTR_ERR(data);
+			goto err;
+		}
+
+		switch (alg) {
+		case Z_EROFS_COMPRESSION_LZ4:
+			ret = z_erofs_load_lz4_config(sb, dsb, data, size);
+			break;
+		default:
+			DBG_BUGON(1);
+			ret = -EFAULT;
+		}
+		kfree(data);
+		if (ret)
+			goto err;
+	}
+err:
+	if (page) {
+		unlock_page(page);
+		put_page(page);
+	}
+	return ret;
+}
+#else
+static int erofs_load_compr_cfgs(struct super_block *sb,
+				 struct erofs_super_block *dsb)
+{
+	if (dsb->u1.available_compr_algs) {
+		erofs_err(sb, "try to load compressed fs when compression is disabled");
+		return -EINVAL;
+	}
+	return 0;
+}
+#endif
+
 static int erofs_read_superblock(struct super_block *sb)
 {
 	struct erofs_sb_info *sbi;
@@ -166,6 +296,12 @@ static int erofs_read_superblock(struct super_block *sb)
 	if (!check_layout_compatibility(sb, dsb))
 		goto out;
 
+	sbi->sb_size = 128 + dsb->sb_extslots * EROFS_SB_EXTSLOT_SIZE;
+	if (sbi->sb_size > EROFS_BLKSIZ) {
+		erofs_err(sb, "invalid sb_extslots %u (more than a fs block)",
+			  sbi->sb_size);
+		goto out;
+	}
 	sbi->blocks = le32_to_cpu(dsb->blocks);
 	sbi->meta_blkaddr = le32_to_cpu(dsb->meta_blkaddr);
 #ifdef CONFIG_EROFS_FS_XATTR
@@ -189,7 +325,10 @@ static int erofs_read_superblock(struct super_block *sb)
 	}
 
 	/* parse on-disk compression configurations */
-	ret = z_erofs_load_lz4_config(sb, dsb, NULL, 0);
+	if (erofs_sb_has_compr_cfgs(sbi))
+		ret = erofs_load_compr_cfgs(sb, dsb);
+	else
+		ret = z_erofs_load_lz4_config(sb, dsb, NULL, 0);
 out:
 	kunmap(page);
 	put_page(page);