On Thu, Feb 10, 2011 at 11:02:50PM -0800, Hugh Dickins wrote:

There is a separate little issue here, Andrea.

Although we went to some trouble for bad_page() to take the page out
of circulation yet let the system continue, your VM_BUG_ON(!PageBuddy)
inside __ClearPageBuddy(page), from two callsites in bad_page(), is
turning it into a fatal error when CONFIG_DEBUG_VM.

I see what you mean. Of course it is only a problem after bad_page
already triggered.... but then it trigger an BUG_ON instead of only a
bad_page.

You could that only MM developers switch CONFIG_DEBUG_VM=y, and they
would like bad_page() to be fatal; maybe, but if so we should do that
as an intentional patch, rather than as an unexpected side-effect ;)

Fedora kernels are built with CONFIG_DEBUG_VM, all my kernels runs
with CONFIG_DEBUG_VM too, so we want it to be as "production" as
possible, and we don't want DEBUG_VM to decrease any reliability (only
to increase it of course).

I noticed this a few days ago, but hadn't quite decided whether just to
remove the VM_BUG_ON, or move it to __ClearPageBuddy's third callsite,
or... doesn't matter much.

I do also wonder if PageBuddy would better be _mapcount -something else:
if we've got a miscounted page (itself unlikely of course), there's a
chance that its _mapcount will be further decremented after it has been
freed: whereupon it will go from -1 to -2, PageBuddy at present.  The
special avoidance of PageBuddy being that it can pull a whole block of
pages into misuse if its mistaken.

Agreed. What about the below?

=====
Subject: mm: PageBuddy cleanups

From: Andrea Arcangeli <redacted>

bad_page could VM_BUG_ON(!PageBuddy(page)) inside __ClearPageBuddy(). I prefer
to keep the VM_BUG_ON for safety and to add a if to solve it.

Change the _mapcount value indicating PageBuddy from -2 to -1024 for more
robusteness against page_mapcount() undeflows.

Signed-off-by: Andrea Arcangeli <redacted>
Reported-by: Hugh Dickins <hughd@google.com>
---

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f6385fc..fa16ba0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -402,16 +402,22 @@ static inline void init_page_count(struct page *page)
 /*
  * PageBuddy() indicate that the page is free and in the buddy system
  * (see mm/page_alloc.c).
+ *
+ * PAGE_BUDDY_MAPCOUNT_VALUE must be <= -2 but better not too close to
+ * -2 so that an underflow of the page_mapcount() won't be mistaken
+ * for a genuine PAGE_BUDDY_MAPCOUNT_VALUE.
  */
+#define PAGE_BUDDY_MAPCOUNT_VALUE (-1024*1024)
+
 static inline int PageBuddy(struct page *page)
 {
-	return atomic_read(&page->_mapcount) == -2;
+	return atomic_read(&page->_mapcount) == PAGE_BUDDY_MAPCOUNT_VALUE;
 }
 
 static inline void __SetPageBuddy(struct page *page)
 {
 	VM_BUG_ON(atomic_read(&page->_mapcount) != -1);
-	atomic_set(&page->_mapcount, -2);
+	atomic_set(&page->_mapcount, PAGE_BUDDY_MAPCOUNT_VALUE);
 }
 
 static inline void __ClearPageBuddy(struct page *page)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a873e61..8aac134 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -286,7 +286,9 @@ static void bad_page(struct page *page)
 
 	/* Don't complain about poisoned pages */
 	if (PageHWPoison(page)) {
-		__ClearPageBuddy(page);
+		/* __ClearPageBuddy VM_BUG_ON(!PageBuddy(page)) */
+		if (PageBuddy(page))
+			__ClearPageBuddy(page);
 		return;
 	}
 

@@ -317,7 +319,8 @@ static void bad_page(struct page *page)
 	dump_stack();
 out:
 	/* Leave bad fields for debug, except PageBuddy could make trouble */
-	__ClearPageBuddy(page);
+	if (PageBuddy(page)) /* __ClearPageBuddy VM_BUG_ON(!PageBuddy(page)) */
+		__ClearPageBuddy(page);
 	add_taint(TAINT_BAD_PAGE);
 }
 

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