linux-kernel - [PATCH] x86/mm/pat: let level meaningful even NULL return in, lookup_address_in

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Date:	Tue, 14 Jul 2015 20:46:06 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	linux-kernel@...r.kernel.org
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, bp@...e.de, jgross@...e.com, mcgrof@...e.com,
	decui@...rosoft.com, ross.zwisler@...ux.intel.com,
	sfr@...b.auug.org.au, toshi.kani@...com,
	"mnipxh@....com" <mnipxh@....com>
Subject: [PATCH] x86/mm/pat: let level meaningful even NULL return in, lookup_address_in_pgd


If pmd or pud is not set, we may set a wrong page mapping level.

Base knowledge:
1) If we have passed pgd_none() check, then current level is at least
PG_LEVEL_1G.
2) If we have passed pud_large() ||!pud_present() check, then current
level is at least PG_LEVEL_2M.

This patch does not change the behavior when a non-NULL value is
returned.

Let's take bask knowledge 2 as an example.
We know *address* belongs to *pud*, however for some reasons *pmd* is
NULL. For example, this address has no physical pages mapped. What we
could benefit from this patch are below:
1) We can walk memory range easier.
If addressA passed to lookup_address(), and NULL returned. We can pass
addressA + level_to_size(level) to lookup_address() in next loop.
...
if (!pte) {
	/* level_to_size has not been implemented in upstream*/
	address += level_to_size(level);
	continue;
}
...
2) keep same behavior because level is set to PG_LEVEL_4K even when pte
is NULL.

So let *level* meaningful even lookup_address_in_pgd return NULL by
moving the assignment before pud/pmd_offset.

Signed-off-by: Pan Xinhui <xinhuix.pan@...el.com>
---
 arch/x86/mm/pageattr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 727158c..a887704 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -336,19 +336,19 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
 	if (pgd_none(*pgd))
 		return NULL;
 
+	*level = PG_LEVEL_1G;
 	pud = pud_offset(pgd, address);
 	if (pud_none(*pud))
 		return NULL;
 
-	*level = PG_LEVEL_1G;
 	if (pud_large(*pud) || !pud_present(*pud))
 		return (pte_t *)pud;
 
+	*level = PG_LEVEL_2M;
 	pmd = pmd_offset(pud, address);
 	if (pmd_none(*pmd))
 		return NULL;
 
-	*level = PG_LEVEL_2M;
 	if (pmd_large(*pmd) || !pmd_present(*pmd))
 		return (pte_t *)pmd;
 
-- 
1.9.1
--
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