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Lab: xv6 lazy page allocation
pdos.csail.mit.edu/6.S081/2020…
新的 2020 版哦。
$ git fetch
$ git checkout lazy
$ make clean
Eliminate allocation from sbrk()
这道题就是把 sys_sbrk 里的 growproc 调用删了，等用到的时候再去分配内存。如果是空间减小，要取消分配。
uint64
sys_sbrk(void)
{
int addr;
int n;
struct proc *p = myproc(); //(+)
if(argint(0, &n) < 0)
return -1;
addr = p->sz; // old sz
p->sz += n;
if (n < 0) { // 空间减小: 取消分配
uvmdealloc(p->pagetable, addr, p->sz);
}
return addr;
}
Lazy allocation
在 vm.c 里面实现惰性分配（莫忘在 defs.h 中声明函数）：
#include "spinlock.h" //(+)
#include "proc.h" //(+)
// lazy allocation memory va for proc p: handle page-fault.
// return allocated memory (pa), 0 for failed
uint64 lazyalloc(struct proc * p, uint64 va){
if(va >= p->sz || va < PGROUNDUP(p->trapframe->sp)){
return 0;
}
char * mem;
uint64 a = PGROUNDDOWN(va);
mem = kalloc();
if(mem == 0){
return 0;
}
memset(mem, 0, PGSIZE);
if(mappages(p->pagetable, a, PGSIZE, (uint64)mem, PTE_W|PTE_X|PTE_R|PTE_U) != 0){
kfree(mem);
return 0;
}
return (uint64)mem;
}
在 usertrap (trap.c) 里处理缺页错误，尝试惰性分配（掉上面写的那个函数，失败就杀掉进程）：
void
usertrap(void)
{
...
if(r_scause() == 8){
// system call
...
} else if((which_dev = devintr()) != 0){
// ok
} else if((r_scause() == 13) || (r_scause() == 15)){ // page fault: (+)
if (lazyalloc(myproc(), r_stval()) <= 0) {
p->killed = 1;
}
} else {
...
}
...
}
最后改一点点细节，把各种缺页会爆出的 panic 干掉（vm.c里）。以前这些情况正常是不会发生的，但现在惰性分配会带来缺页，所以要告诉操作系统遇到这些事情时 don't panic，继续往下跑就行了：
（这里的代码我懒得改结构就上goto了，但你应该去改if结构，而不是goto）
// vm.c
...
//(+)
uint64
walkaddr(pagetable_t pagetable, uint64 va)
{
...
if(pte == 0)
goto lzac;
if((*pte & PTE_V) == 0)
goto lzac;
if((*pte & PTE_U) == 0)
goto lzac;
pa = PTE2PA(*pte);
if (0) {
lzac:
if ((pa = lazyalloc(myproc(), va)) <= 0)
pa = 0;
}
return pa;
}
int
mappages(pagetable_t pagetable, uint64 va, uint64 size, uint64 pa, int perm)
{
...
for(;;){
...
if(*pte & PTE_V)
// panic("remap");
;
...
}
void
uvmunmap(pagetable_t pagetable, uint64 va, uint64 npages, int do_free)
{
...
for(a = va; a < va + npages*PGSIZE; a += PGSIZE){
if((pte = walk(pagetable, a, 0)) == 0) {
// panic("uvmunmap: walk");
continue;
}
if((*pte & PTE_V) == 0) {
// panic("uvmunmap: not mapped");
continue;
}
...
}
}
int
uvmcopy(pagetable_t old, pagetable_t new, uint64 sz)
{
...
for(i = 0; i < sz; i += PGSIZE){
if((pte = walk(old, i, 0)) == 0){
// panic("uvmcopy: pte should exist");
continue;
}
if((*pte & PTE_V) == 0){
// panic("uvmcopy: page not present");
continue;
}
...
}
...
}
测试
写的时候可以按题目跑这些测试：
xv6-labs-2020 $ make qemu
$ echo hi
$ lazytests
$ usertests
diff
详细的 git diff (可以用来做 patch 喔):
diff --git a/kernel/defs.h b/kernel/defs.h
index 4b9bbc0..7ff16c4 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -171,6 +171,7 @@ uint64 walkaddr(pagetable_t, uint64);
int copyout(pagetable_t, uint64, char *, uint64);
int copyin(pagetable_t, char *, uint64, uint64);
int copyinstr(pagetable_t, char *, uint64, uint64);
+uint64 lazyalloc(struct proc * p, uint64 va);
// plic.c
void plicinit(void);
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index e8bcda9..b799722 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -43,12 +43,15 @@ sys_sbrk(void)
{
int addr;
int n;
+ struct proc *p = myproc();
if(argint(0, &n) < 0)
return -1;
- addr = myproc()->sz;
- if(growproc(n) < 0)
- return -1;
+ addr = p->sz; // old sz
+ p->sz += n;
+ if (n < 0) {
+ uvmdealloc(p->pagetable, addr, p->sz);
+ }
return addr;
}
diff --git a/kernel/trap.c b/kernel/trap.c
index a63249e..fc08231 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -67,6 +67,11 @@ usertrap(void)
syscall();
} else if((which_dev = devintr()) != 0){
// ok
+ } else if((r_scause() == 13) || (r_scause() == 15)){ // page fault
+ // lazy allocation
+ if (lazyalloc(myproc(), r_stval()) <= 0) {
+ p->killed = 1;
+ }
} else {
printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
printf(" sepc=%p stval=%p\n", r_sepc(), r_stval());
diff --git a/kernel/vm.c b/kernel/vm.c
index bccb405..f3235c3 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -5,6 +5,8 @@
#include "riscv.h"
#include "defs.h"
#include "fs.h"
+#include "spinlock.h"
+#include "proc.h"
/*
* the kernel's page table.
@@ -102,12 +104,19 @@ walkaddr(pagetable_t pagetable, uint64 va)
pte = walk(pagetable, va, 0);
if(pte == 0)
- return 0;
+ goto lzac;
if((*pte & PTE_V) == 0)
- return 0;
+ goto lzac;
if((*pte & PTE_U) == 0)
- return 0;
+ goto lzac;
pa = PTE2PA(*pte);
+
+ if (0) {
+lzac:
+ if ((pa = lazyalloc(myproc(), va)) <= 0)
+ pa = 0;
+ }
+
return pa;
}
@@ -157,7 +166,8 @@ mappages(pagetable_t pagetable, uint64 va, uint64 size, uint64 pa, int perm)
if((pte = walk(pagetable, a, 1)) == 0)
return -1;
if(*pte & PTE_V)
- panic("remap");
+ // panic("remap");
+ ;
*pte = PA2PTE(pa) | perm | PTE_V;
if(a == last)
break;
@@ -180,10 +190,14 @@ uvmunmap(pagetable_t pagetable, uint64 va, uint64 npages, int do_free)
panic("uvmunmap: not aligned");
for(a = va; a < va + npages*PGSIZE; a += PGSIZE){
- if((pte = walk(pagetable, a, 0)) == 0)
- panic("uvmunmap: walk");
- if((*pte & PTE_V) == 0)
- panic("uvmunmap: not mapped");
+ if((pte = walk(pagetable, a, 0)) == 0) {
+ // panic("uvmunmap: walk");
+ continue;
+ }
+ if((*pte & PTE_V) == 0) {
+ // panic("uvmunmap: not mapped");
+ continue;
+ }
if(PTE_FLAGS(*pte) == PTE_V)
panic("uvmunmap: not a leaf");
if(do_free){
@@ -314,10 +328,14 @@ uvmcopy(pagetable_t old, pagetable_t new, uint64 sz)
char *mem;
for(i = 0; i < sz; i += PGSIZE){
- if((pte = walk(old, i, 0)) == 0)
- panic("uvmcopy: pte should exist");
- if((*pte & PTE_V) == 0)
- panic("uvmcopy: page not present");
+ if((pte = walk(old, i, 0)) == 0){
+ // panic("uvmcopy: pte should exist");
+ continue;
+ }
+ if((*pte & PTE_V) == 0){
+ // panic("uvmcopy: page not present");
+ continue;
+ }
pa = PTE2PA(*pte);
flags = PTE_FLAGS(*pte);
if((mem = kalloc()) == 0)
@@ -440,3 +458,40 @@ copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max)
return -1;
}
}
+
+
+// lazy allocation memory va for proc p: handle page-fault.
+// return allocated memory (pa), 0 for failed
+uint64 lazyalloc(struct proc * p, uint64 va){
+#define lazyalloc_debug 0
+#define lazyalloc_warn(info) { \
+ printf("lazyalloc(): %s", info); \
+ printf(" scause %p pid=%d\n", r_scause(), p->pid); \
+ printf(" sepc=%p stval=%p\n", r_sepc(), r_stval()); \
+}
+ if(va >= p->sz || va < PGROUNDUP(p->trapframe->sp)){
+ #if lazyalloc_debug
+ lazyalloc_warn("vm addr higher then any allocated with sbrk\n");
+ #endif
+ return 0;
+ }
+ char * mem;
+ uint64 a = PGROUNDDOWN(va);
+ mem = kalloc();
+ if(mem == 0){
+ #if lazyalloc_debug
+ lazyalloc_warn("kalloc() == 0\n");
+ #endif
+ return 0;
+ }
+ memset(mem, 0, PGSIZE);
+ if(mappages(p->pagetable, a, PGSIZE, (uint64)mem, PTE_W|PTE_X|PTE_R|PTE_U) != 0){
+ #if lazyalloc_debug
+ lazyalloc_warn("mappages() != 0\n");
+ #endif
+ kfree(mem);
+ return 0;
+ }
+
+ return (uint64)mem;
+}
结果
最后 make grade 看成绩了：
xv6-labs-2020 $ make grade
...
== Test running lazytests ==
$ make qemu-gdb
(7.7s)
== Test lazy: map ==
lazy: map: OK
== Test lazy: unmap ==
lazy: unmap: OK
== Test usertests ==
$ make qemu-gdb
(145.8s)
== Test usertests: pgbug ==
usertests: pgbug: OK
== Test usertests: sbrkbugs ==
usertests: sbrkbugs: OK
== Test usertests: argptest ==
usertests: argptest: OK
== Test usertests: sbrkmuch ==
... OK ...
== Test usertests: dirfile ==
usertests: dirfile: OK
== Test usertests: iref ==
usertests: iref: OK
== Test usertests: forktest ==
usertests: forktest: OK
== Test time ==
time: OK
Score: 119/119
CDFMLR
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