Re: Nick's core remove PageReserved broke vmware...

[Hugh Dickins <hugh@veritas.com>]

On Tue, 8 Nov 2005, Michael S. Tsirkin wrote:
> 
> Hugh, did you have something like the following in mind
> (this is only boot-tested and only on x86-64)?

Yes, that looks pretty good to me, a few comments below.
Only another twenty or so architectures to go ;)

(I had been imagining VM_DONTCOPY plus another flag to say set by the
user: better your way, we would like to merge these vmas, and VM_DONTCOPY
is in that peculiar list of special flags that prevent merging.)

> Hmm, maybe MADV_INHERIT and MADV_DONT_INHERIT would be better names,

You're right, and it would be a good choice, except that MAP_INHERIT on
some OSes has a particular meaning (about inheriting across an exec),
so I think avoid confusion with that.  MADV_DONTFORK and MADV_DOFORK?
Accompanied by VM_DONTFORK?

> since the copy is only dont one write ...

"only done on write"?

> Index: linux-2.6.14-dontcopy/include/linux/mm.h
> ===================================================================
> --- linux-2.6.14-dontcopy.orig/include/linux/mm.h	2005-11-08 23:24:58.000000000 +0200
> +++ linux-2.6.14-dontcopy/include/linux/mm.h	2005-11-08 23:25:09.000000000 +0200
> @@ -154,6 +154,7 @@ extern unsigned int kobjsize(const void 
>  					/* Used by sys_madvise() */
>  #define VM_SEQ_READ	0x00008000	/* App will access data sequentially */
>  #define VM_RAND_READ	0x00010000	/* App will not benefit from clustered reads */
> +#define VM_UDONTCOPY	0x02000000      /* App wants to set VM_DONTCOPY */

Please place it where you'd expect to find it by value.

>  #define VM_DONTCOPY	0x00020000      /* Do not copy this vma on fork */
>  #define VM_DONTEXPAND	0x00040000	/* Cannot expand with mremap() */
> Index: linux-2.6.14-dontcopy/include/asm-x86_64/mman.h
> ===================================================================
> --- linux-2.6.14-dontcopy.orig/include/asm-x86_64/mman.h	2005-11-08 23:19:35.000000000 +0200
> +++ linux-2.6.14-dontcopy/include/asm-x86_64/mman.h	2005-11-08 23:19:46.000000000 +0200
> @@ -36,6 +36,8 @@
>  #define MADV_SEQUENTIAL	0x2		/* read-ahead aggressively */
>  #define MADV_WILLNEED	0x3		/* pre-fault pages */
>  #define MADV_DONTNEED	0x4		/* discard these pages */
> +#define MADV_DONTCOPY	0x30		/* dont inherit across fork */
> +#define MADV_DOCOPY	0x31		/* do inherit across fork */
>  
>  /* compatibility flags */
>  #define MAP_ANON	MAP_ANONYMOUS

I think that's probably a good idea, to choose a range away from the rest.
But I'm not quite sure: anyone familiar with adding APIs listening?

Hugh