linux-kernel - Re: [patch] mm: fix build warning for uninitialized value

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Date:	Mon, 5 Nov 2012 13:36:04 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Haggai Eran <haggaie@...lanox.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch] mm: fix build warning for uninitialized value

do_wp_page() sets mmun_called if mmun_start and mmun_end were initialized 
and, if so, may call mmu_notifier_invalidate_range_end() with these 
values.  This doesn't prevent gcc from emitting a build warning though:

mm/memory.c: In function ‘do_wp_page’:
mm/memory.c:2530: warning: ‘mmun_start’ may be used uninitialized in this function
mm/memory.c:2531: warning: ‘mmun_end’ may be used uninitialized in this function

It's much easier to initialize the variables to impossible values and do a 
simple comparison to determine if they were initialized to remove the bool 
entirely.

Signed-off-by: David Rientjes <rientjes@...gle.com>
---
 mm/memory.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2527,9 +2527,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	int ret = 0;
 	int page_mkwrite = 0;
 	struct page *dirty_page = NULL;
-	unsigned long mmun_start;	/* For mmu_notifiers */
-	unsigned long mmun_end;		/* For mmu_notifiers */
-	bool mmun_called = false;	/* For mmu_notifiers */
+	unsigned long mmun_start = 0;	/* For mmu_notifiers */
+	unsigned long mmun_end = 0;	/* For mmu_notifiers */
 
 	old_page = vm_normal_page(vma, address, orig_pte);
 	if (!old_page) {
@@ -2708,8 +2707,7 @@ gotten:
 		goto oom_free_new;
 
 	mmun_start  = address & PAGE_MASK;
-	mmun_end    = (address & PAGE_MASK) + PAGE_SIZE;
-	mmun_called = true;
+	mmun_end    = mmun_start + PAGE_SIZE;
 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
 
 	/*
@@ -2778,7 +2776,7 @@ gotten:
 		page_cache_release(new_page);
 unlock:
 	pte_unmap_unlock(page_table, ptl);
-	if (mmun_called)
+	if (mmun_end > mmun_start)
 		mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 	if (old_page) {
 		/*