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

[William Tambe <tambewilliam@gmail.com>]

Yes, I have a good case, but my case may not sound interesting until you 
see it working.

Ok, I am developing a dynamic memory allocation routine which takes 
direct advantage of the ability of a machine to use Virtual Memory to 
make everything look contiguous and fast.

And it just doesn't make sens to have mmap() map ANONYMOUS shared memory 
and mremap() not to expand it and make the expanded area available.

I am not a 100% sure, but I don't think correcting its behavior would 
cause a problem.

Would you happen to know how I can work around that issue for now, and 
make writing in an expended area not to generate a Bus error?

Sincerely,
William Tambe


Hugh Dickins wrote:
> 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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