linux-kernel - Re: [RFC][PATCH] vmalloc: simplify vread()/vwrite()

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Date:	Thu, 7 Jan 2010 11:57:36 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Nick Piggin <npiggin@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>
Subject: Re: [RFC][PATCH] vmalloc: simplify vread()/vwrite()

On Thu, 7 Jan 2010 10:50:54 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:
 
> > > The changes are:
> > > - remove the vmlist walk and rely solely on vmalloc_to_page()
> > > - replace the VM_IOREMAP check with (page && page_is_ram(pfn))
> > > 
> > > The VM_IOREMAP check is introduced in commit d0107eb07320b for per-cpu
> > > alloc. Kame, would you double check if this change is OK for that
> > > purpose?
> > > 
> > I think VM_IOREMAP is for avoiding access to device configuration area and
> > unexpected breakage in device. Then, VM_IOREMAP are should be skipped by
> > the caller. (My patch _just_ moves the avoidance of callers to vread()/vwrite())
> 
> "device configuration area" is not RAM, so testing of RAM would be
> able to skip them?
>
Sorry, that's an area what I'm not sure. 
But, page_is_ram() implementation other than x86 seems not very safe...
(And it seems that it's not defiend in some archs.)

> > 
> > > The page_is_ram() check is necessary because kmap_atomic() is not
> > > designed to work with non-RAM pages.
> > > 
> > I think page_is_ram() is not a complete method...on x86, it just check
> > e820's memory range. checking VM_IOREMAP is better, I think.
> 
> (double check) Not complete or not safe?
> 
I think not-safe because e820 doesn't seem to be updated.

> EFI seems to not update e820 table by default.  Ying, do you know why?
> 

I hope all this kinds can be fixed by kernel/resource.c in generic way....
Now, each archs have its own.

> > > Even for a RAM page, we don't own the page, and cannot assume it's a
> > > _PAGE_CACHE_WB page. So I wonder whether it's necessary to do another
> > > patch to call reserve_memtype() before kmap_atomic() to ensure cache
> > > consistency?
> > > 
> > > TODO: update comments accordingly
> > > 
> > 
> > BTW, f->f_pos problem on 64bit machine still exists and this patch is still
> > hard to test. I stopped that because anyone doesn't show any interests.
> 
> I'm using your patch :)
> 
> I feel most inconfident on this patch, so submitted it for RFC first.
> I'll then submit a full patch series including your f_pos fix.
> 
Thank you, it's helpful.

Thanks,
-Kame

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