Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmasof a mergeable VMA

[Linus Torvalds]

On Fri, 9 Apr 2010, Johannes Weiner wrote:
> +	/*
> +	 * 1. case: vma and next are split parts of one root vma.
> +	 * Their anon_vma_chain is equal and we can drop that of next.
> +	 *
> +	 * 2. case: one vma was instantiated as mergeable with the
> +	 * other one and inherited the other one's primary anon_vma as
> +	 * the singleton in its chain.
> +	 *
> +	 * If next came after vma, vma's chain is already an unstrict
> +	 * superset of next's and we can treat it like case 1.
> +	 *
> +	 * If vma has the singleton chain, we have to copy next's
> +	 * unique anon_vmas over.
> +	 */

This comment makes my head hurt. In fact, the whole anon_vma thing hurts 
my head.

Can we have some better high-level documentation on what happens for all 
the cases.

 - split (mprotect, or munmap in the middle):

	anon_vma_clone: the two vma's will have the same anon_vma, and the 
	anon_vma chains will be equivalent. 

 - merge (mprotect that creates a mergeable state):

	anon_vma_merge: we're supposed to have a anon_vma_chain that is 
	a superset of the two chains of the merged entries.

 - fork:

	anon_vma_fork: each new vma will have a _new_ anon_vma as it's 
	primary one, and will link to the old primary trough the 
	anon_vma_chain. It's doing this with a anon_vma_clone() followed 
	by adding an entra entry to the new anon_vma, and setting 
	vma->anon_vma to the new one.

 - create/mmap:

	anon_vma_prepare: find a mergeable anon_vma and use that as a 
	singleton, because the other entries on the anon_vma chain won't 
	matter, since they cannot be associated with any pages associated 
	with the newly created vma..

Correct?

Quite frankly, just looking at that, I can't see how we get to your rules. 
At least not trivially. Especially with multiple merges, I don't see 
how "singleton" is such a special case.

		Linus
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