Re: [patch 09/11] sparc32: use PUD rather than PGD to get PMD in srmmu_nocache_init()

[Mike Rapoport <rppt@linux.ibm.com>]

On Fri, May 22, 2020 at 10:23:09PM -0700, Andrew Morton wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> Subject: sparc32: use PUD rather than PGD to get PMD in srmmu_nocache_init()
> 
> The kbuild test robot reported the following warning:
> 
> arch/sparc/mm/srmmu.c: In function 'srmmu_nocache_init':
> >> arch/sparc/mm/srmmu.c:300:9: error: variable 'pud' set but not used
> >> [-Werror=unused-but-set-variable]
> 300 |  pud_t *pud;
> 
> This warning is caused by misprint in the page table traversal in
> srmmu_nocache_init() function which accessed a PMD entry using PGD rather
> than PUD.
> 
> Since sparc32 has only 3 page table levels, the PGD and PUD are
> essentially the same and usage of __nocache_fix() removed the type
> checking.
> 
> Use PUD for the consistency and to silence the compiler warning.

Unfortunately, this fixes a compile warning but breaks the boot :(

Since pgd, p4d and pud are eesentially the same thing and pgd_offset()
and p4d_offset() are no-ops, the __nocache_fix() should be done only at
pud level.

The correcteted patch is below, boot tested with qemu-systems-sparc.

> Link: http://lkml.kernel.org/r/20200520132005.GM1059226@linux.ibm.com
> Fixes: 7235db268a2777bc38 ("sparc32: use pgtable-nopud instead of 4level-fixup")
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> Reported-by: kbuild test robot <lkp@intel.com>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Anatoly Pugachev <matorola@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---

From 9b8b3214f7f079864dca0c6a7b684ab442eeb437 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.ibm.com>
Date: Wed, 20 May 2020 15:39:22 +0300
Subject: [PATCH] sparc32: use PUD rather than PGD to get PMD in  srmmu_nocache_init()

The kbuild test robot reported the following warning:

arch/sparc/mm/srmmu.c: In function 'srmmu_nocache_init':
>> arch/sparc/mm/srmmu.c:300:9: error: variable 'pud' set but not used
>> [-Werror=unused-but-set-variable]
300 |  pud_t *pud;

This warning is caused by misprint in the page table traversal in
srmmu_nocache_init() function which accessed a PMD entry using PGD rather
than PUD.

Since pgd, p4d and pud are eesentially the same thing and pgd_offset()
and p4d_offset() are no-ops, the __nocache_fix() should be done only at
pud level.

Remove __nocache_fix() for p4d_offset() and pud_offset() and use it only
for PUD and lower levels.

Link: http://lkml.kernel.org/r/20200520132005.GM1059226@linux.ibm.com
Fixes: 7235db268a2777bc38 ("sparc32: use pgtable-nopud instead of 4level-fixup")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reported-by: kbuild test robot <lkp@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Anatoly Pugachev <matorola@gmail.com>
Cc: <stable@vger.kernel.org>
---
 arch/sparc/mm/srmmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/sparc/mm/srmmu.c b/arch/sparc/mm/srmmu.c
index 6cb1ea2d2b5c..3d9690d940a3 100644
--- a/arch/sparc/mm/srmmu.c
+++ b/arch/sparc/mm/srmmu.c
@@ -302,9 +302,9 @@ static void __init srmmu_nocache_init(void)
 
 	while (vaddr < srmmu_nocache_end) {
 		pgd = pgd_offset_k(vaddr);
-		p4d = p4d_offset(__nocache_fix(pgd), vaddr);
-		pud = pud_offset(__nocache_fix(p4d), vaddr);
-		pmd = pmd_offset(__nocache_fix(pgd), vaddr);
+		p4d = p4d_offset(pgd, vaddr);
+		pud = pud_offset(p4d, vaddr);
+		pmd = pmd_offset(__nocache_fix(pud), vaddr);
 		pte = pte_offset_kernel(__nocache_fix(pmd), vaddr);
 
 		pteval = ((paddr >> 4) | SRMMU_ET_PTE | SRMMU_PRIV);
-- 
2.26.2