mkfs_ext2: explain why 0.5G+ images are a bit different

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/util-linux/mkfs_ext2.c b/util-linux/mkfs_ext2.c
index be9e36332bd899fa5669a49f8bd0cd5e52a41387..de4c8b6c6c4eabc9c81f522e2014c393379fcdd5 100644 (file)
--- a/util-linux/mkfs_ext2.c
+++ b/util-linux/mkfs_ext2.c
@@ -309,20 +309,23 @@ int mkfs_ext2_main(int argc UNUSED_PARAM, char **argv)
 
        {
                // N.B. e2fsprogs does as follows!
-               // ninodes is the total number of inodes (files) in the file system
-               uint32_t ninodes = ((uint64_t) nblocks_full * blocksize) / bytes_per_inode;
                uint32_t overhead, remainder;
+               // ninodes is the max number of inodes in this filesystem
+               uint32_t ninodes = ((uint64_t) nblocks_full * blocksize) / bytes_per_inode;
                if (ninodes < EXT2_GOOD_OLD_FIRST_INO+1)
                        ninodes = EXT2_GOOD_OLD_FIRST_INO+1;
                inodes_per_group = div_roundup(ninodes, ngroups);
                // minimum number because the first EXT2_GOOD_OLD_FIRST_INO-1 are reserved
                if (inodes_per_group < 16)
                        inodes_per_group = 16;
-
-               // a block group can have no more than 8*blocksize inodes
+               // a block group can't have more inodes than blocks
                if (inodes_per_group > blocks_per_group)
                        inodes_per_group = blocks_per_group;
                // adjust inodes per group so they completely fill the inode table blocks in the descriptor
+//incompatibility on images >= 0.5GB:
+//difference in sizeof(*inode) sometimes
+//results in slightly bigger inodes_per_group here
+//compared to standard mke2fs:
                inodes_per_group = (div_roundup(inodes_per_group * sizeof(*inode), blocksize) * blocksize) / sizeof(*inode);
                // make sure the number of inodes per group is a multiple of 8
                inodes_per_group &= ~7;
@@ -345,6 +348,7 @@ int mkfs_ext2_main(int argc UNUSED_PARAM, char **argv)
                ////) {
                ////    bb_error_msg_and_die("way small device");
                ////}
+
                // Standard mke2fs uses 50. Looks like a bug in our calculation
                // of "remainder" or "overhead" - we don't match standard mke2fs
                // when we transition from one group to two groups
@@ -414,6 +418,9 @@ int mkfs_ext2_main(int argc UNUSED_PARAM, char **argv)
        sb = xzalloc(1024);
        STORE_LE(sb->s_rev_level, 1); // revision 1 filesystem
        STORE_LE(sb->s_magic, EXT2_SUPER_MAGIC);
+//incompatibility:
+//on images > 0.5GB, standard mke2fs uses 256 byte inodes.
+//we always use 128 byte ones:
        STORE_LE(sb->s_inode_size, sizeof(*inode));
        STORE_LE(sb->s_first_ino, EXT2_GOOD_OLD_FIRST_INO);
        STORE_LE(sb->s_log_block_size, blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE);