linux-kernel - Re: Re : vm: weird behaviour when munmapping

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Message-ID: <38b2ab8a0611200330w17a84994ne3a0eed11ae4485c@mail.gmail.com>
Date:	Mon, 20 Nov 2006 12:30:10 +0100
From:	"Francis Moreau" <francis.moro@...il.com>
To:	"Hugh Dickins" <hugh@...itas.com>
Cc:	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org
Subject: Re: Re : vm: weird behaviour when munmapping

On 11/18/06, Hugh Dickins <hugh@...itas.com> wrote:
> On Fri, 17 Nov 2006, Francis Moreau wrote:
> > On Fri, 2006-11-17 at 14:12 +0000, moreau francis wrote:
> > > Peter Zijlstra wrote:
> > >
> > > The new object is the one allocated using:
> > > new = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
> >
> > Of course but at this point the choice of the new VMA is already made
> > by the caller. So in our case do_munmap() decided that B is the new
> > one as you said. But I still don't see why...
>
> split_vma decides which address range will use the newly allocated
> vm_area_struct in such a way as to suit its own convenience, and

again I don't agree. I would say that do_munmap() decides which
address range will use the new allocated vma object. split_vma() get
this information through its parameter named "new_below".

> >
> > And as I said previously it will end up by calling consecutively:
> >
> >        vma->vm_ops->open(B)
> >        vma->vm_ops->close(B)
>
> You are attaching too much significance to the current address
> of the vma which is passed to your driver in open and close.
> As mmap.c splits and merges vmas, in response to system calls
> unmapping and mapping, those addresses will change.
>
> The important thing is the info contained within the vma: perhaps
> your underlying complaint is that your driver is not getting as
> much info as it wants about what's happening?
>

not really. I'm not writing a real driver. I just try to understand
how vma things work in Linux. Therefore I just wrote a dumb driver
which has modified vma open/close method in order to detect how these
method are called.

I end up to see "open(B), close(B)" sequence when unmapping a part of
the dumb device that I found strange. I think that "open(A') close(B)"
can give more information to the driver and reflect that B is unmapped
and A' is still mapped and becomes the new mapped area.
But it's may be just me...

thanks

Francis
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