分类: LINUX
2013-06-09 17:48:46
原文地址:linux内存管理之sys_brk实现分析(续) 作者:xgr180
unmap_region是整个收缩过程中的核心,它主要完成相应项表项的修改,具体映射页框的释放
代码如下:
static void unmap_region(struct mm_struct *mm,
struct vm_area_struct *vma,
struct vm_area_struct *prev,
unsigned long start,
unsigned long end)
{
struct mmu_gather *tlb;
unsigned long nr_accounted = 0;
lru_add_drain();
tlb = tlb_gather_mmu(mm, 0);
//断开具体的vma映射
unmap_vmas(&tlb, mm, vma, start, end, &nr_accounted, NULL);
vm_unacct_memory(nr_accounted);
//在x86平台上,is_hugepage_only_range()恒为零
if (is_hugepage_only_range(start, end - start))
hugetlb_free_pgtables(tlb, prev, start, end);
else
//因为删除了一些映射,会造成一个页表空闲的情况,回收页表项所占的空间
free_pgtables(tlb, prev, start, end);
tlb_finish_mmu(tlb, start, end);
}
unmap_vmas用来释放pte所映射的页面。代码如下:
//参数说明:
//mm:进程描述符 vma:要删除的起始vma start_addr:要删除的线性区的起始地址
// end_addr:要删除的线性区的结束地址 details:在调用的时候置为了NULL ^_^
int unmap_vmas(struct mmu_gather **tlbp, struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long start_addr,
unsigned long end_addr, unsigned long *nr_accounted,
struct zap_details *details)
{
unsigned long zap_bytes = ZAP_BLOCK_SIZE;
unsigned long tlb_start = 0; /* For tlb_finish_mmu */
int tlb_start_valid = 0;
int ret = 0;
int atomic = details && details->atomic;
//遍历要删除的vma链表
for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) {
unsigned long start;
unsigned long end;
//确定要断开映射的起始地址跟结束地址
start = max(vma->vm_start, start_addr);
if (start >= vma->vm_end)
continue;
end = min(vma->vm_end, end_addr);
if (end <= vma->vm_start)
continue;
if (vma->vm_flags & VM_ACCOUNT)
*nr_accounted += (end - start) >> PAGE_SHIFT;
ret++;
//while循环开始断开start到end的所有被映射的页框,在足够的情况下一次释放zap_bytes
while (start != end) {
unsigned long block;
if (!tlb_start_valid) {
tlb_start = start;
tlb_start_valid = 1;
}
//在条件编译下is_vm_hugetlb_page()为空
if (is_vm_hugetlb_page(vma)) {
block = end - start;
unmap_hugepage_range(vma, start, end);
} else {
//block:要释放的线性区大小
block = min(zap_bytes, end - start);
//断开从start到start + block之间的映射
unmap_page_range(*tlbp, vma, start,
start + block, details);
}
//更新起始地址
start += block;
zap_bytes -= block;
if (!atomic && need_resched()) {
int fullmm = tlb_is_full_mm(*tlbp);
tlb_finish_mmu(*tlbp, tlb_start, start);
cond_resched_lock(&mm->page_table_lock);
*tlbp = tlb_gather_mmu(mm, fullmm);
tlb_start_valid = 0;
}
if ((long)zap_bytes > 0)
continue;
zap_bytes = ZAP_BLOCK_SIZE;
}
}
return ret;
}
跟进unmap_page_range():
static void unmap_page_range(struct mmu_gather *tlb,
struct vm_area_struct *vma, unsigned long address,
unsigned long end, struct zap_details *details)
{
pgd_t * dir;
BUG_ON(address >= end);
//取得页目录
dir = pgd_offset(vma->vm_mm, address);
tlb_start_vma(tlb, vma);
//断开pgd项对应的pmd
do {
zap_pmd_range(tlb, dir, address, end - address, details);
//加上一个pgd大小,并对应PGD_SIZE
address = (address + PGDIR_SIZE) & PGDIR_MASK;
dir++;
} while (address && (address < end));
//x86为空函数,忽略
tlb_end_vma(tlb, vma);
}
转入zap_pmd_range():
static void zap_pmd_range(struct mmu_gather *tlb,
pgd_t * dir, unsigned long address,
unsigned long size, struct zap_details *details)
{
pmd_t * pmd;
unsigned long end, pgd_boundary;
//页目录没有映射
if (pgd_none(*dir))
return;
//无效
if (unlikely(pgd_bad(*dir))) {
pgd_ERROR(*dir);
pgd_clear(dir);
return;
}
//找到起始的pmd
pmd = pmd_offset(dir, address);
end = address + size;
pgd_boundary = ((address + PGDIR_SIZE) & PGDIR_MASK);
if (pgd_boundary && (end > pgd_boundary))
end = pgd_boundary;
do {
//根据pmd找到pte
(tlb, pmd, address, end - address, details);
address = (address + PMD_SIZE) & PMD_MASK;
pmd++;
} while (address && (address < end));
}
继续跟进zap_pte_range():
static void zap_pte_range(struct mmu_gather *tlb,
pmd_t *pmd, unsigned long address,
unsigned long size, struct zap_details *details)
{
unsigned long offset;
pte_t *ptep;
//pmd没有映射页面
if (pmd_none(*pmd))
return;
//无效情况
if (unlikely(pmd_bad(*pmd))) {
pmd_ERROR(*pmd);
pmd_clear(pmd);
return;
}
ptep = pte_offset_map(pmd, address);
offset = address & ~PMD_MASK;
if (offset + size > PMD_SIZE)
size = PMD_SIZE - offset;
size &= PAGE_MASK;
if (details && !details->check_mapping && !details->nonlinear_vma)
details = NULL;
for (offset=0; offset < size; ptep++, offset += PAGE_SIZE) {
pte_t pte = *ptep;
//pte没有映射页面
if (pte_none(pte))
continue;
//相应的页在主存中
if (pte_present(pte)) {
struct page *page = NULL;
//将pte映射的物理地址转换为页面号
unsigned long pfn = pte_pfn(pte);
//如果页面号合法,则转换为相应的page,如果页面被保留(不可以断开映射),page置``````````````//为NULL
if (pfn_valid(pfn)) {
page = pfn_to_page(pfn);
if (PageReserved(page))
//Reserverd:留给内核使用或者没有使用
page = NULL;
}
//函数调用时,details为NULL。略过这部份代码 ^_^
if (unlikely(details) && page) {
/*
* unmap_shared_mapping_pages() wants to
* invalidate cache without truncating:
* unmap shared but keep private pages.
*/
if (details->check_mapping &&
details->check_mapping != page->mapping)
continue;
/*
* Each page->index must be checked when
* invalidating or truncating nonlinear.
*/
if (details->nonlinear_vma &&
(page->index < details->first_index ||
page->index > details->last_index))
continue;
}
//清除pte值,并返回原来的pte值
pte = ptep_get_and_clear(ptep);
tlb_remove_tlb_entry(tlb, ptep, address+offset);
//如果page 为NULL,说明不需要释放page
if (unlikely(!page))
continue;
if (unlikely(details) && details->nonlinear_vma
&& linear_page_index(details->nonlinear_vma,
address+offset) != page->index)
set_pte(ptep, pgoff_to_pte(page->index));
//如果页面项为脏,置page为脏
if (pte_dirty(pte))
set_page_dirty(page);
if (pte_young(pte) && !PageAnon(page))
mark_page_accessed(page);
tlb->freed++;
page_remove_rmap(page);
//在tlb_remove_page里判断page的引用计数,如果没有引用了
//调用free_page_and_swap_cache将页面释放
tlb_remove_page(tlb, page);
continue;
}
if (unlikely(details))
continue;
//如果页表项所映射的数据被交换到了磁盘,释放相关数据
if (!pte_file(pte))
free_swap_and_cache(pte_to_swp_entry(pte));
//清除pte映射
pte_clear(ptep);
}
pte_unmap(ptep-1);
}
通过上面的分析可以看到,内核是如何通过线性地址从pgd找到pte再释放相关页面的。到这一步,注意到,只是释放了pte所映射的页框,所以,可能会造成有很多pte项没有映射的状态,这部份pte所占的空间其实是可以回收的。
它是在free_pgtables()函数中完成的。代码如下:
static void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *prev,
unsigned long start, unsigned long end)
{
//PGD_SIZE大小对齐
unsigned long first = start & PGDIR_MASK;
//向上增加一个PGD_SIZE,last参数还会在接下来进行调整的
unsigned long last = end + PGDIR_SIZE - 1;
unsigned long start_index, end_index;
struct mm_struct *mm = tlb->mm;
//调整first与last参数的值
if (!prev) {
prev = mm->mmap;
if (!prev)
goto no_mmaps;
if (prev->vm_end > start) {
if (last > prev->vm_start)
last = prev->vm_start;
goto no_mmaps;
}
}
for (;;) {
struct vm_area_struct *next = prev->vm_next;
if (next) {
if (next->vm_start < start) {
prev = next;
continue;
}
if (last > next->vm_start)
last = next->vm_start;
}
if (prev->vm_end > first)
first = prev->vm_end + PGDIR_SIZE - 1;
break;
}
no_mmaps:
//非法退出
if (last < first)
return;
//first所在的页目录
start_index = pgd_index(first);
if (start_index < FIRST_USER_PGD_NR)
start_index = FIRST_USER_PGD_NR;
//last所在页目录
end_index = pgd_index(last);
if (end_index > start_index) {
//将页目录start_index到end_index中所映射的pte所占空间释放掉
clear_page_tables(tlb, start_index, end_index - start_index);
flush_tlb_pgtables(mm, first & PGDIR_MASK, last & PGDIR_MASK);
}
}
在研究代码之前,我们不妨先来思考几个问题:
1:一次要释放的多长的地址区间才合适呢?
以i32二级映射关系为例来说明一下:
虽然pte在线性地址中只占有10位,但是实际上为pte分配内存的时候,却分配了一个页。也就是说,pgd中每一项所指向的pte占一个页面.即2^10的pte项占一个页面。而pte本身映射2^12大小的线性地址。所以,要释放一个pte内框所需的地址长度为2^10*2^12 = 2^22 = PGD_SIZE
I32的三级映射也类似
2:prev指向的是什么?
调用这个函数的时候,prev指向的是什么区域的vma呢?
刚开始的时候:
detach_vmas_to_be_unmapped后:
看上面可以看出: clear_page_tables中,要操作的线性地址即为prev,prev->next之间的空洞线性地址。理解了这点之后,上面的代码就变得很简单了^_^
三:用户空间的伸展
先回顾一下sys_brk的代码:
asmlinkage unsigned long sys_brk(unsigned long brk)
{
……
……
//前一部份是用户空间的收缩
/* Check against rlimit.. */
//不能超过数据段上限
rlim = current->rlim[RLIMIT_DATA].rlim_cur;
if (rlim < RLIM_INFINITY && brk - mm->start_data > rlim)
goto out;
/* Check against existing mmap mappings. */
//伸展空间已经有映射了
if (find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE))
goto out;
/* Ok, looks good - let it rip. */
//执行具体的伸展过程
if (do_brk(oldbrk, newbrk-oldbrk) != oldbrk)
goto out;
set_brk:
//设置新边界
mm->brk = brk;
out:
retval = mm->brk;
up_write(&mm->mmap_sem);
return retval;
}
在这有一个值得注意的地方:
find_vma_intersection()的实现如下:
//判断进程的地址空间是否与给定的地址区间相交叉
static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * mm, unsigned long start_addr, unsigned long end_addr)
{
//找到第一个结束地址大于addr的vma
struct vm_area_struct * vma = find_vma(mm,start_addr);
//判断vma是否是给定地址区间有交叉
if (vma && end_addr <= vma->vm_start)
vma = NULL;
return vma;
}
那为什么sys_brk中
find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE)调用中,newbrk为什么要加上PAGE_SIZE呢?
这是因为newbrk 与oldbrk已经是经过页框对齐后的地址:如下
newbrk = PAGE_ALIGN(brk);
oldbrk = PAGE_ALIGN(mm->brk);
而且,每个vma的起始地址跟长度都是与页框对齐的(参考ULK3).注意到find_vma_intersection()判断是否交替的时候带有一个’=’.也就是判断newbrk的下一个页框是否在进程的线性区中
接着往下看,经过判断之后,就会进入到do_brk():
unsigned long do_brk(unsigned long addr, unsigned long len)
{
struct mm_struct * mm = current->mm;
struct vm_area_struct * vma, * prev;
unsigned long flags;
struct rb_node ** rb_link, * rb_parent;
pgoff_t pgoff = addr >> PAGE_SHIFT;
//长度按页框对齐,不过在我们这个流程来说,这个步骤是没必要的
//因为start 与 end都与页框对齐,end – start肯定也是与页框对齐的
len = PAGE_ALIGN(len);
if (!len)
return addr;
//有效性判断
if ((addr + len) > TASK_SIZE || (addr + len) < addr)
return -EINVAL;
//VM_LOCKED: 页被锁住不能被交换出去
if (mm->def_flags & VM_LOCKED) {
unsigned long locked, lock_limit;
locked = mm->locked_vm << PAGE_SHIFT;
lock_limit = current->rlim[RLIMIT_MEMLOCK].rlim_cur;
locked += len;
if (locked > lock_limit && !capable(CAP_IPC_LOCK))
return -EAGAIN;
}
/*
* Clear old maps. this also does some error checking for us
*/
munmap_back:
//sys_brk的流程会进入到这个if吗???
vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
if (vma && vma->vm_start < addr + len) {
if (do_munmap(mm, addr, len))
return -ENOMEM;
goto munmap_back;
}
//判断是否超过了限制
if ((mm->total_vm << PAGE_SHIFT) + len
> current->rlim[RLIMIT_AS].rlim_cur)
return -ENOMEM;
if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;
//判断系统是否有足够的内存
if (security_vm_enough_memory(len >> PAGE_SHIFT))
return -ENOMEM;
flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
//判断是否可以合并
//如果可以合并,就将基合并为一个VMA区
if (vma_merge(mm, prev, addr, addr + len, flags,
NULL, NULL, pgoff, NULL))
goto out;
//不可以合并,新建一个VMA
vma = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
if (!vma) {
vm_unacct_memory(len >> PAGE_SHIFT);
return -ENOMEM;
}
memset(vma, 0, sizeof(*vma));
//设值VMA的值
vma->vm_mm = mm;
vma->vm_start = addr;
vma->vm_end = addr + len;
vma->vm_pgoff = pgoff;
vma->vm_flags = flags;
vma->vm_page_prot = protection_map[flags & 0x
//将新分配的VMA插入到进程的VMA链表
vma_link(mm, vma, prev, rb_link, rb_parent);
out:
mm->total_vm += len >> PAGE_SHIFT;
if (flags & VM_LOCKED) {
mm->locked_vm += len >> PAGE_SHIFT;
//如果定义了LOCKED。就为其分配内存
make_pages_present(addr, addr + len);
}
return addr;
}
make_pages_present()其实就是为每一个线性区模拟了一个缺页异常,然后再由缺页异常程序为之分配内存。
若vm flag没有带VM_LOCKED的时候,它只是为进程分配了一个可以使用的线性地址,以后要访问这个地址的时候,就会产生缺页异常,具体关于缺页异常的处理,我们在下一节接着分析
四:总结
我们在前面分析过了vfree()的实现。还记得vfree()只是释放了内存页表项所映射的物理内存,而在进程管理的时候,sys_brk收缩线性区的时候,它不仅释放了内表所映射的物理内存还把空间页表项。PMD所占的内存释放掉了。内核这样处理是为了效率考虑的。
另外,sys_brk在扩展线性区的时候,仅分配了一个允许进程使用的合法的线性地址,等到真正要使用的时候再给其映射具体的内存,这在操作系统设计里也叫请求调页。等到下节分析缺页异常的时候,再来详细讨论