linux-kernel - Re: [PATCH v2 0/4] x86, mm: Handle large PAT bit in pud/pmd interfaces

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Date:	Tue, 21 Jul 2015 08:52:00 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	akpm@...ux-foundation.org, bp@...en8.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, jgross@...e.com,
	konrad.wilk@...cle.com, elliott@...com
Subject: Re: [PATCH v2 0/4] x86, mm: Handle large PAT bit in pud/pmd
 interfaces

On Tue, 2015-07-21 at 10:05 +0200, Ingo Molnar wrote:
> * Toshi Kani <toshi.kani@...com> wrote:
> 
> > The PAT bit gets relocated to bit 12 when PUD and PMD mappings are 
> > used.
> > This bit 12, however, is not covered by PTE_FLAGS_MASK, which is 
> > corrently
> > used for masking pfn and flags for all cases.
> > 
> > Patch 1/4-2/4 make changes necessary for patch 3/4 to use 
> > P?D_PAGE_MASK.
> > 
> > Patch 3/4 fixes pud/pmd interfaces to handle the PAT bit when PUD and 
> > PMD
> > mappings are used.
> > 
> > Patch 3/4 fixes /sys/kernel/debug/kernel_page_tables to show the PAT 
> > bit
> > properly.
> > 
> > Note, the PAT bit is first enabled in 4.2-rc1 with WT mappings.
> 
> Are patches 1-3 only needed to fix /sys/kernel/debug/kernel_page_tables 
> output, or 
> are there other things fixed as well? The patches do not tell us any of 
> that information ...

Patch 3 (and patch 1-2 needed for patch 3) fixes multiple pud/pmd
interfaces to work properly with _PAGE_PAT_LARGE bit set.  Because pmem is
the only module that can create a range with this bit set with large page
WT maps in 4.2, this issue has not been exposed other than the case in
kernel_page_tables fixed by patch 4.  Since there can be other cases in
future, all patches should go to 4.2 to prevent them to happen.  There is
no issue in 4.1 & older since they cannot set the bit.

Thanks,
-Toshi
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