linux-kernel - Re: "x86-64, mm: Put early page table high" causes crash on Xen

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Date:	Tue, 8 Mar 2011 17:40:51 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: "x86-64, mm: Put early page table high" causes crash on Xen

On Tue, 8 Mar 2011, Yinghai Lu wrote:
> On Tue, Mar 8, 2011 at 4:14 AM, Stefano Stabellini
> <stefano.stabellini@...citrix.com> wrote:
> >> > [    0.000000] DEBUG alloc low page(cf965000, 00001000)
> >>
> >> these are not mapped yet, and need to use early_ioremap.
> >
> > In fact we are mapping them inside alloc_low_page. No need for
> > map_low_page, right?
> 
> alloc_low_page return virtual address, but it can not be used to
> access that position right away.
> so in that case map_low_page is used to convert that virtual address
> to phy address and then to early map address.
 
Are you sure?
I am looking at this piece of code under kernel_physical_mapping_init:


---
        pud = alloc_low_page(&pud_phys);
		last_map_addr = phys_pud_init(pud, __pa(start), __pa(next),
						 page_size_mask);
		unmap_low_page(pud);
---

and the implementaion of alloc_low_page is:

---
static __ref void *alloc_low_page(unsigned long *phys)
{
	unsigned long pfn = pgt_buf_end++;
	void *adr;

	if (after_bootmem) {
		adr = (void *)get_zeroed_page(GFP_ATOMIC | __GFP_NOTRACK);
		*phys = __pa(adr);

		return adr;
	}

	if (pfn >= pgt_buf_top)
		panic("alloc_low_page: ran out of memory");

	adr = early_memremap(pfn * PAGE_SIZE, PAGE_SIZE);
	clear_page(adr);
	*phys  = pfn * PAGE_SIZE;
	return adr;
}
---

It looks to me that map_low_page is never called in this code path.
If I read the code correctly map_low_page is only used to map pagetable
pages that are already mapped (see the debug logging I posted in the
previous email) so it is not useful.