Re: Why don't we make mmap MAP_SHARED with /dev/zero possible?

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Tue, 26 Oct 1999 9:57:48 +0800, fxzhang <fxzhang@chpc.ict.ac.cn>
said:

> static int mmap_zero(struct file * file, struct vm_area_struct * vma)
> {
>         if (vma->vm_flags & VM_SHARED)
>                 return -EINVAL;

> I don't understand why people don't implement it.Yes,in the source,I
> find something like "the shared case is complex",Could someone tell
> me what's the difficulty?As it is a driver,I think it should not be
> too much to concern.

It is not a driver issue --- it is core to the VM.  The VM cannot
handle shared writable anonymous pages.  We're not talking about mmap
pages in this special case: we are talking about normal anonymous data
pages. 

>    Is there any good way to share memory between process at page
> granularity?That is,I can share individual pages between them?
> Threads maybe a subtitue,but there are many things that I don't want
> to share.

SysV shared memory.  "man shmget; man shmop; man shmctl"

--Stephen
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