From 7241838b4cecefb32bad4698e748fc31d008d94d Mon Sep 17 00:00:00 2001 From: Frans Kaashoek Date: Tue, 20 Aug 2019 20:23:18 -0400 Subject: Move labs into 6.828 repo. The lab text isn't dependent on specific xv6 code. Lab submission instructions etc. are likely going to be more MIT 6.828 specific. --- labs/lazy.html | 132 --------------------------------------------------------- 1 file changed, 132 deletions(-) delete mode 100644 labs/lazy.html (limited to 'labs/lazy.html') diff --git a/labs/lazy.html b/labs/lazy.html deleted file mode 100644 index 9d97cab..0000000 --- a/labs/lazy.html +++ /dev/null @@ -1,132 +0,0 @@ - - -Lab: xv6 lazy page allocation - - - - -

Lab: xv6 lazy page allocation

- -

-One of the many neat tricks an O/S can play with page table hardware -is lazy allocation of 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. There are programs that allocate memory but -never use it, for example to implement large sparse arrays. -Sophisticated kernels delay allocation of each page of memory until -the application tries to use that page -- as signaled by a page fault. -You'll add this lazy allocation feature to xv6 in this lab. - -

Part One: Eliminate allocation from sbrk()

- -Your first task is to delete page allocation from the sbrk(n) system -call implementation, which is the function sys_sbrk() in sysproc.c. The -sbrk(n) system call grows the process's memory size by n bytes, and -then returns the start of the newly allocated region (i.e., the old -size). Your new sbrk(n) should just increment the process's size -(myproc()->sz) by n and return the old size. It should not allocate memory --- so you should delete the call to growproc() (but you still need to -increase the process's size!). - -

-Try to guess what the result of this modification will be: what will -break? - -

-Make this modification, boot xv6, and type echo hi to the shell. -You should see something like this: - -

-init: starting sh
-$ echo hi
-usertrap(): unexpected scause 0x000000000000000f pid=3
-            sepc=0x00000000000011dc stval=0x0000000000004008
-va=0x0000000000004000 pte=0x0000000000000000
-panic: unmappages: not mapped
-
- -The "usertrap(): ..." message is from the user trap handler in trap.c; -it has caught an exception that it does not know how to handle. Make -sure you understand why this page fault occurs. The "stval=0x0..04008" -indicates that the virtual address that caused the page fault is -0x4008. - -

Part Two: Lazy allocation

- -Modify the code in trap.c to respond to a page fault from user space -by mapping a newly-allocated page of physical memory at the faulting -address, and then returning back to user space to let the process -continue executing. You should add your code just before -the printf call that produced the "usertrap(): ..." -message. - -

-Hint: look at the printf arguments to see how to find the virtual -address that caused the page fault. - -

-Hint: steal code from allocuvm() in vm.c, which is what sbrk() -calls (via growproc()). - -

-Hint: use PGROUNDDOWN(va) to round the faulting virtual address -down to a page boundary. - -

-Hint: usertrapret() in order to avoid -the printf and the myproc()->killed = 1. - -

-Hint: you'll need to call mappages(). - -

Hint: you can check whether a fault is a page fault by r_scause() - is 13 or 15 in trap(). - -

Hint: modify unmappages() to not free pages that aren't mapped. - -

Hint: if the kernel crashes, look up sepc in kernel/kernel.asm - -

Hint: if you see the error "imcomplete type proc", include "proc.h" - (and "spinlock.h"). - -

Hint: the first test in sbrk() allocates something large, this - should succeed now. - -

-If all goes well, your lazy allocation code should result in echo -hi working. You should get at least one page fault (and thus lazy -allocation) in the shell, and perhaps two. - -

If you have the basics working, now turn your implementation into - one that handles the corner cases too: - -

- -

Run all tests in usertests() to make sure your solution doesn't -break other tests. - -

-

-

Submit: The code that you added to trap.c in a file named hwN.c where N is the homework number as listed on the schedule. -

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