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SubjectRe: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA
On 04/09/2010 07:22 PM, Linus Torvalds wrote:
>
>
> 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?

This is indeed 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.

The trick is in the fact that anon_vma_merge is only called
when vma->anon_vma == vma1->anon_vma.

If the top anon_vmas are different, then anon_vma_merge will
not be called.

This means that VMAs which have recently passed through fork
will not be passed to anon_vma_merge, because their top
anon_vmas are different.

That leaves just the split & create cases, which will be
passed to anon_vma_merge when they are merged.

In case of split, they will have identical anon_vma chains.

In case of create + merge, one of the two VMAs will have
the whole anon_vma chain, while the other one has just
the top anon_vma.


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