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

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Date:	Thu, 7 Jan 2010 13:24:03 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"Huang, Ying" <ying.huang@...el.com>,
	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>
Subject: Re: [RFC][PATCH] vmalloc: simplify vread()/vwrite()

On Thu, Jan 07, 2010 at 11:23:04AM +0800, KAMEZAWA Hiroyuki wrote:
> On Thu, 07 Jan 2010 11:15:41 +0800
> Huang Ying <ying.huang@...el.com> wrote:
> 
> > > > 
> > > > > 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?
> > > 
> > > EFI seems to not update e820 table by default.  Ying, do you know why?
> > 
> > In EFI system, E820 table is constructed from EFI memory map in boot
> > loader, so I think you can rely on E820 table.
> > 
> Yes, we can rely on. But concerns here is that we cannot get any
> information of ioremap via e820 map. 
> 
> But yes,
> == ioremap()
>  140         for (pfn = phys_addr >> PAGE_SHIFT;
>  141                                 (pfn << PAGE_SHIFT) < (last_addr & PAGE_MASK);
>  142                                 pfn++) {
>  143 
>  144                 int is_ram = page_is_ram(pfn);
>  145 
>  146                 if (is_ram && pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn)))
>  147                         return NULL;
>  148                 WARN_ON_ONCE(is_ram);
>  149         }
> ==
> you'll get warned before access if "ram" area is remapped...

Right.

> But, about this patch, it seems that page_is_ram() is not free from architecture
> dependecy.

Yes this is a problem. We can provide a generic page_is_ram() as below.
And could further convert the existing x86 (and others) page_is_ram()
to be resource-based -- since at least for now the e820 table won't be
updated on memory hotplug.

Thanks,
Fengguang
---
 include/linux/ioport.h |    2 ++
 kernel/resource.c      |   18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

--- linux-mm.orig/kernel/resource.c	2010-01-07 12:40:55.000000000 +0800
+++ linux-mm/kernel/resource.c	2010-01-07 13:13:46.000000000 +0800
@@ -297,6 +297,24 @@ int walk_system_ram_range(unsigned long 
 
 #endif
 
+static int __page_is_ram(unsigned long pfn, unsigned long nr_pages, void *arg)
+{
+	int *is_ram = arg;
+
+	*is_ram = 1;
+
+	return 1;
+}
+
+int __attribute__((weak)) page_is_ram(unsigned long pagenr)
+{
+	int is_ram = 0;
+
+	walk_system_ram_range(pagenr, 1, &is_ram, __page_is_ram);
+
+	return is_ram;
+}
+
 /*
  * Find empty slot in the resource tree given range and alignment.
  */
--- linux-mm.orig/include/linux/ioport.h	2010-01-07 13:11:43.000000000 +0800
+++ linux-mm/include/linux/ioport.h	2010-01-07 13:12:37.000000000 +0800
@@ -188,5 +188,7 @@ extern int
 walk_system_ram_range(unsigned long start_pfn, unsigned long nr_pages,
 		void *arg, int (*func)(unsigned long, unsigned long, void *));
 
+extern int page_is_ram(unsigned long pagenr);
+
 #endif /* __ASSEMBLY__ */
 #endif	/* _LINUX_IOPORT_H */
--
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