Re: [tip:x86/mm] resource: Fix generic page_is_ram() for partial RAM pages

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Mar 02, 2010 at 07:45:39AM +0800, Yinghai Lu wrote:
> On 03/01/2010 11:00 AM, tip-bot for Wu Fengguang wrote:
> > Commit-ID:  37b99dd5372cff42f83210c280f314f10f99138e
> > Gitweb:     http://git.kernel.org/tip/37b99dd5372cff42f83210c280f314f10f99138e
> > Author:     Wu Fengguang <fengguang.wu@intel.com>
> > AuthorDate: Mon, 1 Mar 2010 21:55:51 +0800
> > Committer:  H. Peter Anvin <hpa@zytor.com>
> > CommitDate: Mon, 1 Mar 2010 10:18:32 -0800
> > 
> > resource: Fix generic page_is_ram() for partial RAM pages
> > 
> > The System RAM walk shall skip partial RAM pages and avoid calling
> > func() on them. So that page_is_ram() return 0 for a partial RAM page.
> > 
> > In particular, it shall not call func() with len=0.
> > This fixes a boot time bug reported by Sachin and root caused by Thomas:
> > 
> >>>>> WARNING: at arch/x86/mm/ioremap.c:111 __ioremap_caller+0x169/0x2f1()
> >>>>> Hardware name: BladeCenter LS21 -[79716AA]-
> >>>>> Modules linked in:
> >>>>> Pid: 0, comm: swapper Not tainted 2.6.33-git6-autotest #1
> >>>>> Call Trace:
> >>>>> [<ffffffff81047cff>] ? __ioremap_caller+0x169/0x2f1
> >>>>> [<ffffffff81063b7d>] warn_slowpath_common+0x77/0xa4
> >>>>> [<ffffffff81063bb9>] warn_slowpath_null+0xf/0x11
> >>>>> [<ffffffff81047cff>] __ioremap_caller+0x169/0x2f1
> >>>>> [<ffffffff813747a3>] ? acpi_os_map_memory+0x12/0x1b
> >>>>> [<ffffffff81047f10>] ioremap_nocache+0x12/0x14
> >>>>> [<ffffffff813747a3>] acpi_os_map_memory+0x12/0x1b
> >>>>> [<ffffffff81282fa0>] acpi_tb_verify_table+0x29/0x5b
> >>>>> [<ffffffff812827f0>] acpi_load_tables+0x39/0x15a
> >>>>> [<ffffffff8191c8f8>] acpi_early_init+0x60/0xf5
> >>>>> [<ffffffff818f2cad>] start_kernel+0x397/0x3a7
> >>>>> [<ffffffff818f2295>] x86_64_start_reservations+0xa5/0xa9
> >>>>> [<ffffffff818f237a>] x86_64_start_kernel+0xe1/0xe8
> >>>>> ---[ end trace 4eaa2a86a8e2da22 ]---
> >>>>> ioremap reserve_memtype failed -22
> > 
> > The return code is -EINVAL, so it failed in the is_ram check, which is
> > not too surprising
> > 
> >> BIOS-provided physical RAM map:
> >>  BIOS-e820: 0000000000000000 - 000000000009c000 (usable)
> >>  BIOS-e820: 000000000009c000 - 00000000000a0000 (reserved)
> >>  BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
> >>  BIOS-e820: 0000000000100000 - 00000000cffa3900 (usable)
> >>  BIOS-e820: 00000000cffa3900 - 00000000cffa7400 (ACPI data)
> > 
> > The ACPI data is not starting on a page boundary and neither does the
> > usable RAM area end on a page boundary. Very useful !
> > 
> >> ACPI: DSDT 00000000cffa3900 036CE (v01 IBM    SERLEWIS 00001000 INTL 20060912)
> > 
> > ACPI is trying to map DSDT at cffa3900, which results in a check
> > vs. cffa3000 which is the relevant page boundary. The generic is_ram
> > check correctly identifies that as RAM because it's in the usable
> > resource area. The old e820 based is_ram check does not take
> > overlapping resource areas into account. That's why it works.
> > 
> > CC: Sachin Sant <sachinp@in.ibm.com>
> > CC: Thomas Gleixner <tglx@linutronix.de>
> > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > LKML-Reference: <20100301135551.GA9998@localhost>
> > Signed-off-by: H. Peter Anvin <hpa@zytor.com>
> > ---
> >  kernel/resource.c |    9 +++++----
> >  1 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/resource.c b/kernel/resource.c
> > index 03c897f..8f0e3d0 100644
> > --- a/kernel/resource.c
> > +++ b/kernel/resource.c
> > @@ -274,7 +274,7 @@ int walk_system_ram_range(unsigned long start_pfn, unsigned long nr_pages,
> >  		void *arg, int (*func)(unsigned long, unsigned long, void *))
> >  {
> >  	struct resource res;
> > -	unsigned long pfn, len;
> > +	unsigned long pfn, end_pfn;
> >  	u64 orig_end;
> >  	int ret = -1;
> >  
> > @@ -284,9 +284,10 @@ int walk_system_ram_range(unsigned long start_pfn, unsigned long nr_pages,
> >  	orig_end = res.end;
> >  	while ((res.start < res.end) &&
> >  		(find_next_system_ram(&res, "System RAM") >= 0)) {
> > -		pfn = (unsigned long)(res.start >> PAGE_SHIFT);
> > -		len = (unsigned long)((res.end + 1 - res.start) >> PAGE_SHIFT);
> > -		ret = (*func)(pfn, len, arg);
> > +		pfn = (res.start + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > +		end_pfn = (res.end + 1) >> PAGE_SHIFT;
> > +		if (end_pfn > pfn)
> > +		    ret = (*func)(pfn, end_pfn - pfn, arg);
> >  		if (ret)
> >  			break;
> >  		res.start = res.end + 1;
> > --
> 
> wonder if we should trim the ram earlier.

It will actually yield more efficient walk_system_ram_range().

However the problem is, we have to ensure page alignment for _every_
arch in order to optimize away the alignment checks in
walk_system_ram_range(), which makes it less feasible. 

Thanks,
Fengguang