> For the complete documentation index, see [llms.txt](https://richardweiyang-2.gitbook.io/kernel-exploring/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://richardweiyang-2.gitbook.io/kernel-exploring/nei-cun-guan-li/00-index/03-page_table_fault/21-folio_index.md).

# Folio.index

## linear\_page\_index()

```
static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
					const unsigned long address)
{
	pgoff_t pgoff;
	pgoff = (address - vma->vm_start) >> PAGE_SHIFT;
	pgoff += vma->vm_pgoff;
	return pgoff;
}
```

其中vm\_start/vm\_pgoff的含义可以参考[vma单个vma的内容](/kernel-exploring/nei-cun-guan-li/00-index/05-vma.md)中的解释。

对于文件映射，得到的就是对应地址在文件中的实际偏移。 对于匿名映射，得到的正常来说就还是address。

## 设置

```
    __handle_mm_fault
        vmf.pgoff = linear_page_index(vma, address)                   // 匿名：虚拟地址；文件：文件内偏移
        ...
        do_anonymous_page
            folio_add_new_anon_rmap
                __folio_set_anon
                    folio->index = linear_page_index(vma, address)    // 也就是对应的进程虚拟地址
        do_fault
	...
            filemap_fault
                index = vmf->pgoff
                __filemap_get_folio(, index, ) -> __filemap_get_folio_mpol(, index, )

                    index = mapping_align_index(mapping, index);      // index对齐文件系统最小order
                    order = __ffs(index);                             // order是从index对齐地址i中获取的
                    folio = filemap_alloc_folio(, order, );           // 再用order去分配folio,所以隐含了index和order对齐的信息

                    filemap_add_folio(folio, index, )
                        __filemap_add_folio(folio, index, )
                            XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio))
                            folio->index = xas.xa_index;              // 实际上就是文件内偏移
```


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