Incrase file size to around 8MB

Use 10 direct block numbers, 2 indirect block numbers and one double
indirect block number to increase the maximum file size to around 8MB.

Using this scheme, we can address a total of blocks
10 + 2 * 128 + 128 * 128 = 16650 blocks or just over 8MB.

file.h

@@ -21,7 +21,7 @@ struct inode {
   short minor;
   short nlink;
   uint size;
-  uint addrs[NDIRECT+1];
+  uint addrs[NDIRECT+3];
 };
 #define I_BUSY 0x1
 #define I_VALID 0x2

fs.c

@@ -356,18 +356,25 @@ iunlockput(struct inode *ip)
 static uint
 bmap(struct inode *ip, uint bn)
 {
-  uint addr, *a;
+  uint addr, *a, i;
   struct buf *bp;
 
+  // We use 10 direct, two indirect and one double indirect pointer
   if(bn < NDIRECT){
+    // If block number is less than direct pointers available
+    // allocate a disk block if not allocated/available
+    // then return the address of the block
     if((addr = ip->addrs[bn]) == 0)
       ip->addrs[bn] = addr = balloc(ip->dev);
     return addr;
   }
+  // The block number does not fall in the first 10 direct blocks
+  // Substract the number from the passed block number
   bn -= NDIRECT;
 
   if(bn < NINDIRECT){
     // Load indirect block, allocating if necessary.
+    // This is the first indirect block range
     if((addr = ip->addrs[NDIRECT]) == 0)
       ip->addrs[NDIRECT] = addr = balloc(ip->dev);
     bp = bread(ip->dev, addr);
@@ -378,6 +385,39 @@ bmap(struct inode *ip, uint bn)
     }
     brelse(bp);
     return addr;
+  } else if(bn < NINDIRECT * 2){
+    bn -= NINDIRECT;
+    // Load indirect block, allocating if necessary.
+    // This is the second indirect block range
+    if((addr = ip->addrs[NDIRECT+1]) == 0)
+      ip->addrs[NDIRECT+1] = addr = balloc(ip->dev);
+    bp = bread(ip->dev, addr);
+    a = (uint*)bp->data;
+    if((addr = a[bn]) == 0){
+      a[bn] = addr = balloc(ip->dev);
+      log_write(bp);
+    }
+    brelse(bp);
+    return addr;
+  } else {
+    // We are now in the double indirect block range
+    bn -= NINDIRECT * 2;
+    // Load indirect block, allocating if necessary.
+    if((addr = ip->addrs[NDIRECT+2]) == 0)
+      ip->addrs[NDIRECT+2] = addr = balloc(ip->dev);
+    bp = bread(ip->dev, addr);
+    a = (uint*)bp->data;
+    // This last double indirect block should contain addresses to
+    // other blocks and not data. Fig 6.4 of xv6 booklet
+    for (i = 0; i < BSIZE/4; i++){
+      if((addr = a[i]) == 0){
+        a[bn] = addr = balloc(ip->dev);
+        log_write(bp);
+        break;
+      }
+    }
+    brelse(bp);
+    return addr;
   }
 
   panic("bmap: out of range");

fs.h

@@ -19,9 +19,9 @@ struct superblock {
   uint nlog;         // Number of log blocks
 };
 
-#define NDIRECT 12
+#define NDIRECT 10
 #define NINDIRECT (BSIZE / sizeof(uint))
-#define MAXFILE (NDIRECT + NINDIRECT)
+#define MAXFILE (NDIRECT + 2 * NINDIRECT + NINDIRECT * NINDIRECT)
 
 // On-disk inode structure
 struct dinode {
@@ -30,7 +30,7 @@ struct dinode {
   short minor;          // Minor device number (T_DEV only)
   short nlink;          // Number of links to inode in file system
   uint size;            // Size of file (bytes)
-  uint addrs[NDIRECT+1];   // Data block addresses
+  uint addrs[NDIRECT+3];   // Data block addresses
 };
 
 // Inodes per block.