linux-kernel - [PATCH v2] mm/mapping_dirty_helpers: Guard hugepage pud's usage

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Date:   Fri, 9 Apr 2021 12:51:51 -0400
From:   Zack Rusin <zackr@...are.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Hellström <thomas_os@...pmail.org>,
        <linux-mm@...ck.org>
Subject: [PATCH v2] mm/mapping_dirty_helpers: Guard hugepage pud's usage

Mapping dirty helpers have, so far, been only used on X86, but
a port of vmwgfx to ARM64 exposed a problem which results
in a compilation error on ARM64 systems:
mm/mapping_dirty_helpers.c: In function ‘wp_clean_pud_entry’:
mm/mapping_dirty_helpers.c:172:32: error: implicit declaration of function ‘pud_dirty’; did you mean ‘pmd_dirty’? [-Werror=implicit-function-declaration]

This is due to the fact that mapping_dirty_helpers code assumes
that pud_dirty is always defined, which is not the case for
architectures that don't define CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD.

ARM64 arch is a little inconsistent when it comes to PUD
hugepage helpers, e.g. it defines pud_young but not pud_dirty
but regardless of that the core kernel code shouldn't assume
that any of the PUD hugepage helpers are available unless
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD is defined. This
prevents compilation errors whenever one of the drivers
is ported to new architectures.

Signed-off-by: Zack Rusin <zackr@...are.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Hellström (Intel) <thomas_os@...pmail.org>
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
---
 mm/mapping_dirty_helpers.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/mapping_dirty_helpers.c b/mm/mapping_dirty_helpers.c
index b59054ef2e10..b890854ec761 100644
--- a/mm/mapping_dirty_helpers.c
+++ b/mm/mapping_dirty_helpers.c
@@ -165,10 +165,12 @@ static int wp_clean_pud_entry(pud_t *pud, unsigned long addr, unsigned long end,
 		return 0;
 	}

+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 	/* Huge pud */
 	walk->action = ACTION_CONTINUE;
 	if (pud_trans_huge(pudval) || pud_devmap(pudval))
 		WARN_ON(pud_write(pudval) || pud_dirty(pudval));
+#endif

 	return 0;
 }
-- 
2.27.0