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List:       git-commits-head
Subject:    [ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo
From:       Linux Kernel Mailing List <linux-kernel () vger ! kernel ! org>
Date:       2008-08-28 20:02:56
Message-ID: 200808282002.m7SK2uUC006974 () hera ! kernel ! org
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Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e80d6a248298721e0ec2cac150c539d8378577d8
Commit:     e80d6a248298721e0ec2cac150c539d8378577d8
Parent:     f1bcf7e3e734ea8713e08fbc3409f8bf26ec418f
Author:     Mel Gorman <mel@csn.ul.ie>
AuthorDate: Thu Aug 14 11:10:14 2008 +0100
Committer:  Russell King <rmk+kernel@arm.linux.org.uk>
CommitDate: Wed Aug 27 20:09:28 2008 +0100

    [ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo
    
    Ordinarily, memory holes in flatmem still have a valid memmap and is safe
    to use. However, an architecture (ARM) frees up the memmap backing memory
    holes on the assumption it is never used. /proc/pagetypeinfo reads the
    whole range of pages in a zone believing that the memmap is valid and that
    pfn_valid will return false if it is not. On ARM, freeing the memmap breaks
    the page->zone linkages even though pfn_valid() returns true and the kernel
    can oops shortly afterwards due to accessing a bogus struct zone *.
    
    This patch lets architectures say when FLATMEM can have holes in the
    memmap. Rather than an expensive check for valid memory, /proc/pagetypeinfo
    will confirm that the page linkages are still valid by checking page->zone
    is still the expected zone. The lookup of page_zone is safe as there is a
    limited range of memory that is accessed when calling page_zone.  Even if
    page_zone happens to return the correct zone, the impact is that the counters
    in /proc/pagetypeinfo are slightly off but fragmentation monitoring is
    unlikely to be relevant on an embedded system.
    
    Reported-by: H Hartley Sweeten <hsweeten@visionengravers.com>
    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
    Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
---
 arch/arm/Kconfig |    5 +++++
 mm/vmstat.c      |   19 ++++++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 4b8acd2..70dba16 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -810,6 +810,11 @@ config OABI_COMPAT
 	  UNPREDICTABLE (in fact it can be predicted that it won't work
 	  at all). If in doubt say Y.
 
+config ARCH_FLATMEM_HAS_HOLES
+	bool
+	default y
+	depends on FLATMEM
+
 config ARCH_DISCONTIGMEM_ENABLE
 	bool
 	default (ARCH_LH7A40X && !LH7A40X_CONTIGMEM)
diff --git a/mm/vmstat.c b/mm/vmstat.c
index b0d08e6..d7826af 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -516,9 +516,26 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
 			continue;
 
 		page = pfn_to_page(pfn);
+#ifdef CONFIG_ARCH_FLATMEM_HAS_HOLES
+		/*
+		 * Ordinarily, memory holes in flatmem still have a valid
+		 * memmap for the PFN range. However, an architecture for
+		 * embedded systems (e.g. ARM) can free up the memmap backing
+		 * holes to save memory on the assumption the memmap is
+		 * never used. The page_zone linkages are then broken even
+		 * though pfn_valid() returns true. Skip the page if the
+		 * linkages are broken. Even if this test passed, the impact
+		 * is that the counters for the movable type are off but
+		 * fragmentation monitoring is likely meaningless on small
+		 * systems.
+		 */
+		if (page_zone(page) != zone)
+			continue;
+#endif
 		mtype = get_pageblock_migratetype(page);
 
-		count[mtype]++;
+		if (mtype < MIGRATE_TYPES)
+			count[mtype]++;
 	}
 
 	/* Print counts */
--
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