linux-kernel - [PATCH] um: asm/page.h: zero out a pte's high value in set_pte

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Date:	Fri, 29 Jan 2016 00:56:48 +0100
From:	Nicolai Stange <nicstange@...il.com>
To:	Richard Weinberger <richard@....at>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] um: asm/page.h: zero out a pte's high value in set_pte_val()

Commit 16da306849d0 ("um: kill pfn_t")
introduced a compile warning for defconfig:

  arch/um/kernel/skas/mmu.c:38:206: warning: right shift count >= width of type
                                    [-Wshift-count-overflow]

Aforementioned patch changes the definition of the phys_to_pfn() macro from

  ((pfn_t) ((p) >> PAGE_SHIFT))

to

  ((p) >> PAGE_SHIFT)

This effectively changes the phys_to_pfn() expansion's type from
unsigned long long to unsigned long.

Through the callchain init_stub_pte()->mk_pte(), the expansion of
phys_to_pfn() is (indirectly) fed into the 'phys' argument of the
pte_set_val(pte, phys, prot) macro, eventually leading to

  (pte).pte_high = (phys) >> 32;

This results in the warning from above.

Since UML only deals with 32 bit addresses, the upper 32 bits from 'phys'
used to be zero anyway.

Zero out the pte value's high part in pte_set_val() in order to get rid
of the offending shift.

Fixes: 16da306849d0 ("um: kill pfn_t")
Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
 arch/um/include/asm/page.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/um/include/asm/page.h b/arch/um/include/asm/page.h
index e13d41c..61e235f 100644
--- a/arch/um/include/asm/page.h
+++ b/arch/um/include/asm/page.h
@@ -46,8 +46,8 @@ typedef struct { unsigned long pgd; } pgd_t;
 			      smp_wmb(); \
 			      (to).pte_low = (from).pte_low; })
 #define pte_is_zero(pte) (!((pte).pte_low & ~_PAGE_NEWPAGE) && !(pte).pte_high)
-#define pte_set_val(pte, phys, prot) \
-	({ (pte).pte_high = (phys) >> 32; \
+#define pte_set_val(pte, phys, prot)				\
+	({ (pte).pte_high = 0;					\
 	   (pte).pte_low = (phys) | pgprot_val(prot); })
 
 #define pmd_val(x)	((x).pmd)
-- 
2.7.0