linux-kernel - Re: [PATCH] s390: Add pmd

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Message-ID: <20121029084956.12b512e6@mschwide>
Date:	Mon, 29 Oct 2012 08:49:56 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] s390: Add pmd_mknotpresent()

On Sun, 28 Oct 2012 14:10:14 +0100
Ingo Molnar <mingo@...nel.org> wrote:

> 
> There's a related problem on s390: other THP implementations 
> have pmd_mknotpresent() while s390 not, resulting in:
> 
>   mm/huge_memory.c:1543:2: error: implicit declaration of function 'pmd_mknotpresent'
> 
> The (untested!) patch below adds the s390 version of this 
> method.
> 
> Gerald, Martin, did I get the S390 details right?

This won't work I'm afraid. S390 uses invalid bits which need to be set to
make an entry not present. Just setting the _SEGMENT_ENTRY_INV bit is not
good enough either, there is _HPAGE_TYPE_NONE to consider. The patch to fix
this for pmd_none & pmd_present just got added to the s390 tree on kernel.org:
https://git.kernel.org/?p=linux/kernel/git/s390/linux.git;a=shortlog;h=refs/heads/fixes

Now, if pmd_mknotpresent is supposed to make the entry invalid so that
pmd_present will return false the function needs to do two things,
1) set the _SEGMENT_ENTRY_INV bit, and 2) clear the _SEGMENT_ENTRY_RO bit.

> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 098fc5a..b820ff1 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1310,6 +1310,12 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
>  	return pmd;
>  }
> 
> +static inline pmd_t pmd_mknotpresent(pmd_t pmd)
> +{
> +	pmd_val(pmd) &= ~_SEGMENT_ENTRY_ORIGIN;
> +	return pmd;
> +}
> +
>  #define __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
>  static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>  					    unsigned long address, pmd_t *pmdp)
> 


-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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