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Date:	Fri, 20 Apr 2007 16:39:52 +0800
From:	"Aubrey Li" <>
To:	"David Howells" <>
Cc:	"Robin Getz" <>,,, "Alan Cox" <>,,,
	"Andrew Morton" <>,
	"Linux Kernel" <>
Subject: Re: [PATCH] CONFIG_PACKET_MMAP should depend on MMU

On 4/20/07, David Howells <> wrote:
> Aubrey Li <> wrote:
> > The patch works properly on my side. But
> > 1) I'm not sure why you re-wrote alloc/free_pg_vec function, doesn't
> > the current implement work for NOMMU? I know you want to allocate the
> > entire data buffer as one contiguous lump, but is it really necessary?
> Yes.  It's not possible to map the whole buffer otherwise.  Think about it!
> mmap() returns _one_ reference address.  In MMU-mode, the non-contiguous
> physical buffers can be made to appear virtually contiguous by fudging the
> page tables and using the MMU.  This is not possible in NOMMU-mode.  The app
> will expect the buffer to be one contiguous lump in its address space, and
> will not be able to locate the other segments of the buffer.

Great explanation, thanks, :-)

> Actually, what I said is not quite true.  It is possible to map the whole
> buffer otherwise: I could lift the restriction that requires that you map the
> whole buffer or not at all, and then userspace could stitch the whole lot
> together itself.  This would then require userspace to be bimodal.
> > 2) So the mapped pages doesn't count into NR_FILE_MAPPED, is it a problem?
> Not really, no - there are no pagetables.
> Furthermore, issuing the PACKET_RX_RING sockopt does the entire allocation.
> Any subsequent mmaps on it have little effect.
> We could do that accounting though if you think it'd be better.  I don't
> suppose it hurts.
as checked in packet_set_ring, buffer size must be a multiple of PAGE_SIZE,
if (unlikely(req->tp_block_size & (PAGE_SIZE - 1)))

So why not use __get_free_pages rather than kmalloc, so that we have
pagetables to count?

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