Understanding the Linux Kernel

14.3. Anticipating Linux 2.4

Much work has been done on the page cache. First of all, the page cache makes use preferably of page frames in high memory. Moreover, Linux 2.4 introduces a new kind of object that represents a file address space : the object refers to a given block of an inode (or of a block device) and includes pointers to both the memory region descriptors mapping the file and the pages containing the file data. The inode object now includes a pointer to the new address space object, and the page cache is indexed by combining the base address of the address space object with the offset inside the file.

The address space object includes methods to read and write a full page of data. These methods take care of inode object management (like updating the file access time), page cache handling, and temporary buffer allocation. This approach leads to a better coupling between the page cache and the buffer cache. Most filesystems can thus use the generic_file_write( ) function (as you'll see in Chapter 15, Linux 2.2 uses this function only for networking filesystems). The synchronization problem between the buffer cache and the page cache has thus been removed: since both read and write operations for regular files make use of the same page cache, it is no longer necessary to synchronize data present in the two caches.

Notice that both the buffer cache and the page cache continue to be used with different purposes. The first acts on disk blocks, the second on pages that ...