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

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Message-ID: <1423609825.1128.24.camel@misato.fc.hp.com>
Date:	Tue, 10 Feb 2015 16:10:25 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Dave Hansen <dave.hansen@...el.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 Tue, 2015-02-10 at 15:13 -0700, Toshi Kani wrote:
> On Tue, 2015-02-10 at 12:51 -0800, Dave Hansen wrote:
> > 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()?
> 
> That's what v1 did, and I found in testing that the PMD mapping code did
> not work when PAE was unset.  I think we need special handling similar
> to one_md_table_init(), which returns pgd as pmd in case of non-PAE.
> ioremap_page_range() does not have such handling and I thought it would
> not be worth adding it.

Actually pud_alloc() and pmd_alloc() should carry pgd in this case...  I
will look into the problem to see why it did not work when PAE was
unset.

Thanks,
-Toshi

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