linux-kernel - Re: [RFC PATCH 3/11] x86, mm, pat: Change reserve

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Message-ID: <1405468387.28702.53.camel@misato.fc.hp.com>
Date:	Tue, 15 Jul 2014 17:53:07 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	plagnioj@...osoft.com, tomi.valkeinen@...com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Bader <stefan.bader@...onical.com>,
	Dave Airlie <airlied@...il.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH 3/11] x86, mm, pat: Change reserve_memtype() to
 handle WT type

On Tue, 2014-07-15 at 16:36 -0700, Andy Lutomirski wrote:
> On Tue, Jul 15, 2014 at 4:10 PM, Toshi Kani <toshi.kani@...com> wrote:
> > On Tue, 2014-07-15 at 12:56 -0700, Andy Lutomirski wrote:
> >> On Tue, Jul 15, 2014 at 12:34 PM, Toshi Kani <toshi.kani@...com> wrote:
> >> > This patch changes reserve_memtype() to handle the new WT type.
> >> > When (!pat_enabled && new_type), it continues to set either WB
> >> > or UC- to *new_type.  When pat_enabled, it can reserve a given
> >> > non-RAM range for WT.  At this point, it may not reserve a RAM
> >> > range for WT since reserve_ram_pages_type() uses the page flags
> >> > limited to three memory types, WB, WC and UC.
> >>
> >> FWIW, last time I looked at this, it seemed like all the fancy
> >> reserve_ram_pages stuff was unnecessary: shouldn't the RAM type be
> >> easy to track in the direct map page tables?
> >
> > Are you referring the direct map page tables as the kernel page
> > directory tables (pgd/pud/..)?
> >
> > I think it needs to be able to keep track of the memory type per a
> > physical memory range, not per a translation, in order to prevent
> > aliasing of the memory type.
> 
> Actual RAM (the lowmem kind, which is all of it on x86_64) is mapped
> linearly somewhere in kernel address space.  The pagetables for that
> mapping could be used as the canonical source of the memory type for
> the ram range in question.
>
> This only works for lowmem, so maybe it's not a good idea to rely on it.

Right.

I think using struct page table for the RAM ranges is a good way for
saving memory, but I wonder how often the RAM ranges are mapped other
than WB...  If not often, reserve_memtype() could simply call
rbt_memtype_check_insert() for all ranges, including RAM.

In this patch, I left using reserve_ram_pages_type() since I do not see
much reason to use WT for RAM, either.

Thanks,
-Toshi

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