Linux Reverse Map

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Read those carefully, you will understand:

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I used to implement the basic PTE-chain based rmap for LegoOS. I can see the downsides of it. I tried to understand the linux rmap before, somehow gave up because I couldn’t fully understand one thing: for a page that is shared among multiple processes’ VMAs, the source code suggests it will always have same offset from the beginning of all VMA (i.e., vm_start). But does it actually works like this for ALL cases? I just think it’s possible that a page is mapped by an VMA which has a slightly different starting address.

I still have doubt about it. But after accepting this assumption, it’s just easy to understand. I will check later on.

The code suggests:

  • The offset of a page is saved in page->index.
  • For anonmouys pages, the page->index is saved by page_set_anon_rmap().
  • When doing rmap walk over multiple VMAs:
  • For anon: unsigned long address = vma_address(page, vma);
  • For file: unsigned long address = vma_address(page, vma);
  • And vma_address() is basically page->index
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    static inline unsigned long
    __vma_address(struct page *page, struct vm_area_struct *vma)
    {
        pgoff_t pgoff = page_to_pgoff(page);
        return vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
    }

Compared to basic PTE-chain based solution, object-based rmap:

The real benefit

  • During page fault, we only need to set page->mapping to point to struct anon_vma, rather than allocating a new structure and insert.

The downside

  • During rmap walk, we need extra computation to walk each VMA’s page table to make sure that the page is actually mapped within this specific VMA.

Adding struct anon_vma is really similar to the idea of reusing address_space, i.e., having a data structure trampoline.

Some more boring details:

  • All pages within a single VMA share just one anon_vma. vma->anon_vma indicates if a VMA has attached or note. Related function is anon_vma_prepare() within do_anonymous_fault() 1.


Yizhou Shan ©️
Created: Jun 16, 2019
Last Updated: Jun 17, 2019