linux-kernel - Re: [PATCH v2 5/7] x86, mm: Support huge KVA mappings on x86

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Date:	Tue, 10 Feb 2015 12:51:04 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	akpm@...ux-foundation.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, arnd@...db.de, linux-mm@...ck.org,
	x86@...nel.org, linux-kernel@...r.kernel.org, Elliott@...com
Subject: Re: [PATCH v2 5/7] x86, mm: Support huge KVA mappings on x86

On 02/10/2015 12:42 PM, Toshi Kani wrote:
> On Tue, 2015-02-10 at 10:59 -0800, Dave Hansen wrote:
>> On 02/09/2015 02:45 PM, Toshi Kani wrote:
>>> Implement huge KVA mapping interfaces on x86.  Select
>>> HAVE_ARCH_HUGE_VMAP when X86_64 or X86_32 with X86_PAE is set.
>>> Without X86_PAE set, the X86_32 kernel has the 2-level page
>>> tables and cannot provide the huge KVA mappings.
>>
>> Not that it's a big deal, but what's the limitation with the 2-level
>> page tables on 32-bit?  We have a 4MB large page size available there
>> and we already use it for the kernel linear mapping.
> 
> ioremap() calls arch-neutral ioremap_page_range() to set up I/O mappings
> with PTEs.  This patch-set enables ioremap_page_range() to set up PUD &
> PMD mappings.  With 2-level page table, I do not think this PUD/PMD
> mapping code works unless we add some special code.

What actually breaks, though?

Can't you just disable the pud code via ioremap_pud_enabled()?

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