/*返回不可写的页的页描述符，如果是COW的第一种情况即zero_pfn可读页，返回NULL，将进入下面的if流程；第二种情况即(父子进程)共享页将正常返回其页描述符*/ old_page = vm_normal_page(vma, address, orig_pte);
if (!old_page) {
/*
* VM_MIXEDMAP !pfn_valid() case
*
* We should not cow pages in a shared writeable mapping.
* Just mark the pages writable as we can't do any dirty
* accounting on raw pfn maps.
*/
/*如果这个vma是可写且共享的，跳到标号reuse，这就不会COW
否则跳到标号gotten*/
if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
(VM_WRITE|VM_SHARED))
goto reuse;
goto gotten;
}
/*
* Take out anonymous pages first, anonymous shared vmas are
* not dirty accountable.
*/
/*下面的if和else流程，都是为了尽可能不进行COW，它们试图进入标号reuse*/
/*如果该页old_page是匿名页(由页描述符的mapping)，
并且只有一个进程使用该页(reuse_swap_page，由页描述符的_mapcount值是否为0)，那么不要搞什么COW了，这个进程就是可以使用该页*/
if (PageAnon(old_page) && !PageKsm(old_page)) {
/*排除其他进程在使用该页的情况，由页描述符的flag*/
if (!trylock_page(old_page)) {
page_cache_get(old_page);
pte_unmap_unlock(page_table, ptl);
lock_page(old_page);
page_table = pte_offset_map_lock(mm, pmd, address,
&ptl);
if (!pte_same(*page_table, orig_pte)) {
unlock_page(old_page);
page_cache_release(old_page);
goto unlock;
}
page_cache_release(old_page);
}
/*判断该页描述符的_mapcount值是否为0*/
reuse = reuse_swap_page(old_page);
unlock_page(old_page);
}
/*如果vma是共享且可写，看看这种情况下有没有机会不COW*/
else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
(VM_WRITE|VM_SHARED))) {
/*
* Only catch write-faults on shared writable pages,
* read-only shared pages can get COWed by
* get_user_pages(.write=1, .force=1).
*/
if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
struct vm_fault vmf;
int tmp;
vmf.virtual_address = (void __user *)(address &
PAGE_MASK);
vmf.pgoff = old_page->index;
vmf.flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
vmf.page = old_page;
/*
* Notify the address space that the page is about to
* become writable so that it can prohibit this or wait
* for the page to get into an appropriate state.
*
* We do this without the lock held, so that it can
* sleep if it needs to.
*/ |
