Copy-on-write memory

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How it works now

Restoring

We have different ideas how to restore COW memory. In a moment we even thought to use KSM (Kernel Shared Memory) for that. As result we found a good way for restoring COW memory (I guess). All VMAs are restored in the same way as they were created. Here are two questions.

  • Which vma-s should be inherited?
  • How to avoid intersections with crtools VMAs?

The first question is not resolved completely. Now a VMA is inherited if a parent has a VMA with the same start and end addresses. This covers 99% of cases, but it doesn't work if a VMA was moved.

The second question is more interesting. Currently crtools reserves continuous space for all private vma-s, then restores all VMAs one by one in this space. Inherited vma-s are moved from a parent space. All vma-s are sorted by start addresses.

Cow.png

In “restorer” all crtools’ vma-s are unmapped and private vma-s are space apart. The complexity of this algorithm is linear. Now it looks simple, but I spent a few hours to find it.

“Complexity is easy; simplicity is difficult. -Georgy Shpagin”
“Everything should be made as simple as possible, but not more simpler. - Albert Einstein”

All VMAs and their contents are restored before forking children, so here is one more item. A parent can change some pages after forking a child, so such pages should be dropped from the child's VMA. For solving this problem bitmaps are used to mark touched pages and madvise() is used to remove extra pages.

One more case is not handled now. COW memory are not restored if a process is reparented to init.

Acronyms

COW - [Copy-on-write]

VMA - Virtual Memory Area

KSM - [Kernel SamePage Merging]