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

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Date:	Tue, 8 Mar 2011 10:54:51 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	"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, Mar 8, 2011 at 9:40 AM, Stefano Stabellini
<stefano.stabellini@...citrix.com> wrote:
> 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.


[    0.000000] kernel direct mapping tables up to 7f750000 @
[0x7f74c000-0x7f74ffff] pre-allocated
[    0.000000] DEBUG map_low_page phys    243a000
[    0.000000] DEBUG map_low_page phys    243e000
[    0.000000] DEBUG alloc_low_page phys   7f74c000
[    0.000000] DEBUG map_low_page phys    243a000
[    0.000000] DEBUG map_low_page phys   7f74c000
[    0.000000] DEBUG alloc_low_page phys   7f74d000
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