linux-kernel - Re: [PATCH 6/7] x86, mm: setup page table from top-down

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Date:	Wed, 10 Oct 2012 10:07:44 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>, Jacob Shin <jacob.shin@....com>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] x86, mm: setup page table from top-down

On Wed, Oct 10, 2012 at 9:38 AM, Stefano Stabellini
<stefano.stabellini@...citrix.com> wrote:
>> -     if (pfn >= pgt_buf_top)
>> -             panic("alloc_low_page: ran out of memory");
>> +     if ((pgt_buf_end + 1) >= pgt_buf_top) {
>> +             unsigned long ret;
>> +             if (min_pfn_mapped >= max_pfn_mapped)
>> +                     panic("alloc_low_page: ran out of memory");
>> +             ret = memblock_find_in_range(min_pfn_mapped << PAGE_SHIFT,
>> +                                     max_pfn_mapped << PAGE_SHIFT,
>> +                                     PAGE_SIZE, PAGE_SIZE);
>> +             if (!ret)
>> +                     panic("alloc_low_page: can not alloc memory");
>> +             memblock_reserve(ret, PAGE_SIZE);
>> +             pfn = ret >> PAGE_SHIFT;
>
> This cannot be right: you are allocating another page to be used as
> pagetable page, outside the range pgt_buf_start-pgt_buf_top.
>
> When that page is going to be hooked into the live pagetable, the kernel
> is going to panic on Xen because the page wasn't marked RO.
>
> If you want to do that you need to tell the Xen subsystem of the new
> page. pagetable_reserve is not the right call, we need a new one (see
> past emails).

ok, will change that interface and call in from alloc_low_page.

how about the pages from BRK, do we need to call xen hooks to mark it as RO?
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