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SubjectRe: vm: weird behaviour when munmapping
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On Fri, 2006-11-17 at 11:31 +0000, moreau francis wrote:
> Hmm, I'm probably missing something but I don't see what. Please be
> nice even if the question is really stupid ;)
>
> I'm looking at mmap.c code and to understand it I decided to implement
> a dumb char device that implement its own foo_mmap() method. In this
> method it defined its own vma ops:
>
> static void foo_vma_open(struct vm_area_struct *vma)
> static void foo_vma_close(struct vm_area_struct *vma)
>
> A dumb application mmap the device in order to make foo_mmap() install
> the vma ops.
>
> mmap(NULL, 16384, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>
> mmap returned 0x2aaae000 for example. Until now, foo_vma_open() and
> foo_vma_close() are not called.
>
> Now I want to unmap the first part of the previous mapping to see how
> vma ops are called. So I did:
>
> munmap(0x2aaae000, 1024);
>
> and here's what happen:
>
> foo_vma_open(vma) is called with:
> vma->vm_start = 0x2aaae000
> vma->vm_end = 0x2aaaf000
>
> foo_vma_close(vma) is called with:
> vma->vm_start = 0x2aaae000
> vma->vm_end = 0x2aaaf000
>
> However I would have expected:
>
> foo_vma_open(vma) is called with:
> vma->vm_start = 0x2aaaf000
> vma->vm_end = 0x2aaab2000
>
> foo_vma_close(vma) is called with:
> vma->vm_start = 0x2aaae000
> vma->vm_end = 0x2aaaf000
>
> Can anybody tell me why I get this behaviour ?
>

http://lwn.net/Kernel/LDD3/

Chapter 15. Section 'Virtual Memory Areas'.

Basically; vm_ops->open() is not called on the first vma. With this
munmap() you split the area in two, and it so happens the new vma is the
lower one.


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