[ 3.5.y.z extended stable ] Patch "mm: Fix PageHead when !CONFIG_PAGEFLAGS_EXTENDED" has been added to staging queue

[Herton Ronaldo Krzesinski]

This is a note to let you know that I have just added a patch titled

    mm: Fix PageHead when !CONFIG_PAGEFLAGS_EXTENDED

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

------

>From 669068d986c86addc2483ef5bb8be35cabccc830 Mon Sep 17 00:00:00 2001
From: Christoffer Dall <cdall at cs.columbia.edu>
Date: Fri, 21 Dec 2012 13:03:50 -0500
Subject: [PATCH] mm: Fix PageHead when !CONFIG_PAGEFLAGS_EXTENDED

commit ad4b3fb7ff9940bcdb1e4cd62bd189d10fa636ba upstream.

Unfortunately with !CONFIG_PAGEFLAGS_EXTENDED, (!PageHead) is false, and
(PageHead) is true, for tail pages.  If this is indeed the intended
behavior, which I doubt because it breaks cache cleaning on some ARM
systems, then the nomenclature is highly problematic.

This patch makes sure PageHead is only true for head pages and PageTail
is only true for tail pages, and neither is true for non-compound pages.

[ This buglet seems ancient - seems to have been introduced back in Apr
  2008 in commit 6a1e7f777f61: "pageflags: convert to the use of new
  macros".  And the reason nobody noticed is because the PageHead()
  tests are almost all about just sanity-checking, and only used on
  pages that are actual page heads.  The fact that the old code returned
  true for tail pages too was thus not really noticeable.   - Linus ]

Signed-off-by: Christoffer Dall <cdall at cs.columbia.edu>
Acked-by:  Andrea Arcangeli <aarcange at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Will Deacon <Will.Deacon at arm.com>
Cc: Steve Capper <Steve.Capper at arm.com>
Cc: Christoph Lameter <cl at linux.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 include/linux/page-flags.h |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index c88d2a9..4dabf0f 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -361,7 +361,7 @@ static inline void ClearPageCompound(struct page *page)
  * pages on the LRU and/or pagecache.
  */
 TESTPAGEFLAG(Compound, compound)
-__PAGEFLAG(Head, compound)
+__SETPAGEFLAG(Head, compound)  __CLEARPAGEFLAG(Head, compound)

 /*
  * PG_reclaim is used in combination with PG_compound to mark the
@@ -373,8 +373,14 @@ __PAGEFLAG(Head, compound)
  * PG_compound & PG_reclaim	=> Tail page
  * PG_compound & ~PG_reclaim	=> Head page
  */
+#define PG_head_mask ((1L << PG_compound))
 #define PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))

+static inline int PageHead(struct page *page)
+{
+	return ((page->flags & PG_head_tail_mask) == PG_head_mask);
+}
+
 static inline int PageTail(struct page *page)
 {
 	return ((page->flags & PG_head_tail_mask) == PG_head_tail_mask);
--
1.7.9.5