linux-kernel - Re: No more bits in vm_area_struct's vm

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Date:	Mon, 28 Sep 2009 12:57:05 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Nigel Cunningham <ncunningham@...a.org.au>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: No more bits in vm_area_struct's vm_flags.

On Mon, 28 Sep 2009 11:36:24 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> On Mon, Sep 28, 2009 at 11:04:50AM +0800, KAMEZAWA Hiroyuki wrote:
 
> > But, yes it implies to add a new argument to several functions in mmap.c
> > and maybe a patch will be ugly.
> > 
> > How about addding this check ?
> > 
> > is_mergeable_vma(...)
> > ....
> >   if (vma->vm_hints)
> > 	return 0;
> > 
> > And not calling vma_merge() at madvice(ACCESS_PATTERN_HINT).
> > 
> > I wonder there are little chances when madice(ACCESS_PATTERN_HINT) is
> > given against mapped-file-vma...
> 
> Me wonder too. The access hints should be rarely used.
> A simple solution is reasonable for them.
> 
> But what if more flags going into vm_hints in future?
> 

yes, that's concern...

But I wonder there are several flags which implies that the vma is not for
merge. as....

#define VM_GROWSDOWN    0x00000100      /* general info on the segment */
#define VM_GROWSUP      0x00000200

or some PFN_MAP flags...
(At least, we can have 4 flags of not-for-merge in vm_hints ;)

Then, Nigel, you have 2 choices I think.

  (1) don't merge if vm_hints is set  or
  (2) pass vm_hints to all __merge() functions.

One of above will be accesptable for stakeholders...
I personally like (1) but just trying (2) may be accepted.

What I dislike is making vm_flags to be long long ;)

Thanks,
-Kame

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