Re: [PATCH 3/4] coredump: ELF-FDPIC: enable to omit anonymous shared memory

From: David Howells <dhowells@redhat.com>
To: Hugh Dickins <hugh@veritas.com>
Cc: Robin Holt <holt@sgi.com>,
	"Kawai, Hidehiro" <hidehiro.kawai.ez@hitachi.com>,
	Andrew Morton <akpm@osdl.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Pavel Machek <pavel@ucw.cz>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	sugita <yumiko.sugita.yf@hitachi.com>,
	Satoshi OSHIMA <soshima@redhat.com>,
	"Hideo AOKI@redhat" <haoki@redhat.com>,
	dhowells@redhat.com
Subject: Re: [PATCH 3/4] coredump: ELF-FDPIC: enable to omit anonymous shared memory
Date: Sat, 03 Mar 2007 14:10:29 +0000	[thread overview]
Message-ID: <29317.1172931029@redhat.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0703021646050.13167@blonde.wat.veritas.com>

Hugh Dickins <hugh@veritas.com> wrote:

> > VMAs are a shared resource under NOMMU conditions.
> 
> That's a disturbing remark.

Why?  No-one complained when I first put up my rewrite patches three years ago.

> Under precisely what NOMMU conditions?

CONFIG_MMU=n.

> I had thought Robin's suggestion very sensible; and throughout mm/
> it has seemed pretty random whether we pass an "mm" argument down
> in addition to "vma", or just take vma->vm_mm at whatever level needs.
> 
> You seem to be suggesting vma->vm_mm is dangerous when CONFIG_NOMMU,

vm_mm is never set to anything other than NULL if CONFIG_MMU=n and it doesn't
seem to be a problem.  I don't think anything in the mm/ directory is left that
looks at vm_mm once MMU support is disabled (in fact I've just checked, and I
can compile with vm_mm #ifdef'd out)

> but we MMU people are scarily unaware of that.

If you're worryied that you can't compile anything for NOMMU, an FRV compiler
is available, and a suitable NOMMU default config can be provided.
Alternatively, you can pick ARM, M68K, ...

> Perhaps you need to put #ifndef CONFIG_NOMMU around vm_mm in struct
> vm_area_struct?

I can if it makes you happier.  It's not strictly necessary, but it does make
the struct smaller which is good.

David