linux-kernel - vm: weird behaviour when munmapping

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Message-ID: <20061117113124.75132.qmail@web23112.mail.ird.yahoo.com>
Date:	Fri, 17 Nov 2006 11:31:24 +0000 (GMT)
From:	moreau francis <francis_moreau2000@...oo.fr>
To:	linux-kernel@...r.kernel.org
Subject: vm: weird behaviour when munmapping

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 ?

thanks

Francis

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