One of the many neat tricks an O/S can play with page table hardware is lazy allocation of user-space heap memory. Xv6 applications ask the kernel for heap memory using the sbrk() system call. In the kernel we've given you, sbrk() allocates physical memory and maps it into the process's virtual address space. It can take a long time for a kernel to allocate and map memory for a large request. Consider, for example, that a gigabyte consists of 262,144 4096-byte pages; that's a huge number of allocations even if each is individually cheap. In addition, some programs allocate more memory than they actually use (e.g., to implement sparse arrays), or allocate memory well in advance of use. To allow sbrk() to complete more quickly in these cases, sophisticated kernels allocate user memory lazily. That is, sbrk() doesn't allocate physical memory, but just remembers which user addresses are allocated and marks those addresses as invalid in the user page table. When the process first tries to use any given page of lazily-allocated memory, the CPU generates a page fault, which the kernel handles by allocating physical memory, zeroing it, and mapping it. You'll add this lazy allocation feature to xv6. Project source: MIT.
giannis@giannis-dell-linux:~/Documents/xv6-project-2022$ make grade
$ make qemu-gdb
pte printout: OK (1.8s)
== Test running lazytests ==
$ make qemu-gdb
(2.8s)
== Test lazy: map ==
lazy: map: OK
== Test lazy: unmap ==
lazy: unmap: OK
== Test usertests ==
$ make qemu-gdb
(366.0s)
== Test usertests: pgbug ==
usertests: pgbug: OK
== Test usertests: sbrkbugs ==
usertests: sbrkbugs: OK
== Test usertests: argptest ==
usertests: argptest: OK
== Test usertests: sbrkmuch ==
usertests: sbrkmuch: OK
== Test usertests: sbrkfail ==
usertests: sbrkfail: OK
== Test usertests: sbrkarg ==
usertests: sbrkarg: OK
== Test usertests: stacktest ==
usertests: stacktest: OK
== Test usertests: execout ==
usertests: execout: OK
== Test usertests: copyin ==
usertests: copyin: OK
== Test usertests: copyout ==
usertests: copyout: OK
== Test usertests: copyinstr1 ==
usertests: copyinstr1: OK
== Test usertests: copyinstr2 ==
usertests: copyinstr2: OK
== Test usertests: copyinstr3 ==
usertests: copyinstr3: OK
== Test usertests: rwsbrk ==
usertests: rwsbrk: OK
== Test usertests: sbrk8000 ==
usertests: sbrk8000: OK
== Test usertests: reparent ==
usertests: reparent: OK
== Test usertests: twochildren ==
usertests: twochildren: OK
== Test usertests: forkfork ==
usertests: forkfork: OK
== Test usertests: forkforkfork ==
usertests: forkforkfork: OK
== Test usertests: exectest ==
usertests: exectest: OK
== Test usertests: sbrkbasic ==
usertests: sbrkbasic: OK
== Test usertests: kernmem ==
usertests: kernmem: OK
== Test usertests: mem ==
usertests: mem: OK
== Test usertests: forktest ==
usertests: forktest: OK
== Test usertests: all tests ==
usertests: all tests: OK
Score: 100/100