The 2.3 development series saw major changes in the way memory management worked. The 2.2 kernel was quite limited in the amount of memory it could use, especially on 32-bit processors. With 2.4, those limits have been lifted; Linux is now able to manage all the memory that the processor is able to address. Some things have had to change to make all this possible; overall, however, the scale of the changes at the API level is surprisingly small.

As we have seen, the 2.4 kernel makes extensive use of pointers to struct page to refer to specific pages in memory. This structure has been present in Linux for a long time, but it was not previously used to refer to the pages themselves; instead, the kernel used logical addresses.

Thus, for example, pte_page returned an unsigned long value instead of struct page *. The virt_to_page macro did not exist at all; if you needed to find a struct page entry you had to go directly to the memory map to get it. The macro MAP_NR would turn a logical address into an index in mem_map; thus, the current virt_to_page macro could be defined (and, in sysdep.h in the sample code, is defined) as follows:

#ifdef MAP_NR #define virt_to_page(page) (mem_map + MAP_NR(page)) #endif

The MAP_NR ...