RE: Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix messageallocation

nickcheng
2008-02-26T04:29:17Z

James,
Appreciate for your answer.
One more question: do you think there are any compatibility issues on
kmalloc/kfree pair?
I just trace the kernel code. It looks like it would not.
But I am not quite sure absolutely.
May I have your comments?
Thank you,

-----Original Message-----
From: James Bottomley [mailto:James.Bottomley@HansenPartnership.com]
Sent: Monday, February 25, 2008 10:39 PM
To: nick.cheng@areca.com.tw
Cc: 'Daniel Drake'; linux-scsi@vger.kernel.org; 'erich'
Subject: RE: Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix
messageallocation

On Mon, 2008-02-25 at 18:47 +0800, nickcheng wrote:
> Sorry, maybe I did not ask distinctly enough.
> I mean if I would like to allocate a memory space from ZONE_DMA for atomic
> context, why can I not use kmalloc(1032, GFP_ATOMIC|GFP_DMA)?
> In case of lack of GFP_DMA, kmalloc would grab the memory from ZONE_HIGH
or
> ZONE_HIGHMEM, isn't it?(I read it from the textbook of Linux Kernel
> Development by Robert Love)

Um, no that's not true at all. GFP_DMA only allocates memory from
ZONE_DMA and fails otherwise. You only need memory from ZONE_DMA if you
cannot address physical memory above 24 bits (the old ISA restriction).
This only applies if you're a standard ISA device and you're going
actually to DMA to the memory in question. Neither of which applies in
the arcmsr case, since you're only using the memory as a copy buffer
within the kernel (it never sees an actual DMA transfer), and arcmsr
doesn't have the ISA restrictions.

> Or the basic is that you don't think it is necessary to allocate a memory
> space from DMA area?
> Please give me some comments.

It's unnecessary because you never DMA to it, but even if you did, since
arcmsr is a non-ISA device (with 64 bit DMA mask falling back to 32) you
can just use ordinary kmalloc'd memory for that (provided you obey the
coherence requirements).

James