linux-kernel - Re: Concerning a post that you made about expandable anonymous shared mappings

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Date:	Mon, 2 Jul 2007 16:33:56 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	William Tambe <tambewilliam@...il.com>
cc:	Stas Sergeev <stsp@...et.ru>, linux-kernel@...r.kernel.org
Subject: Re: Concerning a post that you made about expandable anonymous shared
 mappings

On Fri, 29 Jun 2007, William Tambe wrote:

> I read a post that you made about not being able to expand anonymous shared
> mapping with mremap(). And I am actually having that issue now.

I guess you're referring to the thread at
http://lkml.org/lkml/2004/6/16/155
and you're asking either Stas or me.

> 
> You made the post in 2004 and we are now in 2007. I would like to know if that
> feature was added because the code below always fail with bus error on my
> machine. I use glibc 2.5

You've answered your own question: we did not make the change Stas
suggested, IIRC because I remained a little uneasy with that change
in behaviour, and nobody else spoke up for it.

I haven't given it any thought since then:
do you have a good case for us to reconsider it?

Hugh

> 
> Thank you for helping.
> 
> #define _GNU_SOURCE
> #include <sys/mman.h>
> #include <unistd.h>
> 
> #include <stdio.h>
> 
> main() {
>         void *ptr;
>         if ((ptr=mmap(0, 4096, PROT_READ|PROT_WRITE,
>                 MAP_ANONYMOUS|MAP_SHARED|MAP_GROWSDOWN, 0, 0)) == -1) {
>                 printf("failed to mmap\n");
>                 return;
>         }
> 
>         if ((ptr=mremap(ptr, 4096, 8192, MREMAP_MAYMOVE)) == -1) {
>                 printf("failed to mremap\n");
>                 return;
>         }
> 
>         //why does this failed. I am well in the interval [4096, 8192]
>         *(unsigned int *)(ptr + 4096 + 8)= 10;
> }
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