c,linux,mmap,virtual-memory , Multiple mappings for a physical page


Multiple mappings for a physical page

Question:

Tag: c,linux,mmap,virtual-memory

I want to create a copy-on-write like interface for accessing a mmap()ed file in GNU C. Here is the way it should work:

  1. I will map the file to the address space using mmap(). Doing so, I will have a pointer to a contiguous region of memory which will contain real data.
  2. Using some sort of magic, I will have another part of the address space pointing to the exact same physical pages. In other words, I will have two different addresses to access any physical page on memory for the mmap()ed region.
  3. Once an instruction tries to write to a page using the second mapping, I will change the mapping for that particular page to point to a different physical page (which I will create in a on-demand fashion).
  4. At some point, I will sync the dirty-page with originally mapped page and change the alias to point to the memory-mapped page.

Here is the question: What is the best way to do this?


Answer:

Still not entirely clear on your exact requirements. But here are some options I see.

Let mmap handle the COW for you using MAP_PRIVATE. Then when you are ready to sync just create a regular mmap (or direct file open) to the original file and do your sync with the modified MAP_PRIVATE page.

That doesn't allow you to know whether the MAP_PRIVATE page was actually modified or not. If you want that (e.g. so that you can optimise and not do a sync unless a page has changed) then you can make the MAP_PRIVATE page readonly. On first access a SEGV will occur. Catch that SEGV with a signal handler and re-map the MAP_PRIVATE page to be writeable and internally note it as dirty.

And finally, if you don't want to use MAP_PRIVATE and want full copy control, just don't use MMAP_PRIVATE but do a readonly mapping and signal handler. In the signal handler, allocate some memory, copy the original page and remap the faulting page.

Hope that all makes sense.


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