[PATCH] ARM: convert max_pfn and max_low_pfn to be relative to PFN0

From: Colin Cross
Date: Wed Jun 12 2013 - 22:13:32 EST

On ARM max_pfn and max_low_pfn have always been relative to the
first valid PFN, apparently due to ancient kernels being unable
to properly handle physical memory at addresses other than 0.
A comment was added:
Note: max_low_pfn and max_pfn reflect the number of _pages_ in
the system, not the maximum PFN.
which conflicts with the comment in include/linux/bootmem.h that
says max_pfn is the highest page. Since then, the number of users
of max_pfn has proliferated, and they all seem to assume that
max_pfn is the highest valid pfn. The only user of max_low_pfn
as a number of pfns instead of the highest pfn is kcore, which
conflates max_low_pfn with the size of low memory, but it is not
supported on ARM.

Remove the PHYS_PFN_OFFSET subtraction from max_pfn and max_low_pfn,
and fix up the rest of ARM mm init that adds it back again.

This fixes reading page map counts and flags from /proc/kpagecount
and /proc/kpageflags, which will return a short read when reading
pfns that overlap with max_pfn, and return 0 when reading pfn
max_pfn, making it impossible to read the flags and count for pfn

>From code inspection, I believe this will also improve block device
performance where the bounce limit was set to BLK_BOUNCE_HIGH, which
was bouncing unnecessarily for the top PHYS_PFN_OFFSET pages of low

Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
arch/arm/mm/init.c | 21 ++++++---------------
1 file changed, 6 insertions(+), 15 deletions(-)

Boot tested on 3.4 and filled up all of memory without any issues.

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 9a5cdc0..b4e4051 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -415,16 +415,8 @@ void __init bootmem_init(void)
arm_bootmem_free(min, max_low, max_high);

- /*
- * This doesn't seem to be used by the Linux memory manager any
- * more, but is used by ll_rw_block. If we can get rid of it, we
- * also get rid of some of the stuff above as well.
- *
- * Note: max_low_pfn and max_pfn reflect the number of _pages_ in
- * the system, not the maximum PFN.
- */
- max_low_pfn = max_low - PHYS_PFN_OFFSET;
- max_pfn = max_high - PHYS_PFN_OFFSET;
+ max_low_pfn = max_low;
+ max_pfn = max_high;

@@ -530,7 +522,6 @@ static inline void free_area_high(unsigned long pfn, unsigned long end)
static void __init free_highpages(void)
- unsigned long max_low = max_low_pfn + PHYS_PFN_OFFSET;
struct memblock_region *mem, *res;

/* set highmem page free */
@@ -539,12 +530,12 @@ static void __init free_highpages(void)
unsigned long end = memblock_region_memory_end_pfn(mem);

/* Ignore complete lowmem entries */
- if (end <= max_low)
+ if (end <= max_low_pfn)

/* Truncate partial highmem entries */
- if (start < max_low)
- start = max_low;
+ if (start < max_low_pfn)
+ start = max_low_pfn;

/* Find and exclude any reserved regions */
for_each_memblock(reserved, res) {
@@ -591,7 +582,7 @@ void __init mem_init(void)
extern u32 itcm_end;

- max_mapnr = pfn_to_page(max_pfn + PHYS_PFN_OFFSET) - mem_map;
+ max_mapnr = pfn_to_page(max_pfn) - mem_map;

/* this will put all unused low memory onto the freelists */

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