[PATCH V2] mm, x86, pat: Improve scaling of pat_pagerange_is_ram()

[John Dykstra <jdykstra@cray.com>]

On Wed, 2012-05-16 at 15:46 -0700, Suresh Siddha wrote:
> Instead of duplicating what kernel/resource.c:walk_system_ram_range() is
> already doing, can we just provide a callback that can be used with
> walk_system_ram_range() and see if the expected RAM pages is what the
> callback also sees.

The resulting patch is a bit longer than V1.  
---

Function pat_pagerange_is_ram() scales poorly to large address ranges,
because it probes the resource tree for each page.  On a 2.6 GHz
Opteron, this function consumes 34 ms. for a 1 GB range.  It is called
twice during untrack_pfn_vma(), slowing process cleanup and handicapping
the OOM killer.

This replacement consumes less than 1ms. under the same conditions.

Signed-off-by: John Dykstra <jdykstra@cray.com> on behalf of Cray Inc. 
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
---
 arch/x86/mm/pat.c |   58
++++++++++++++++++++++++++++++++--------------------
 1 files changed, 36 insertions(+), 22 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index f6ff57b..246cce8 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -158,31 +158,45 @@ static unsigned long pat_x_mtrr_type(u64 start,
u64 end, unsigned long req_type)
 	return req_type;
 }
 
+struct pagerange_is_ram_state {
+	unsigned long		cur_pfn;
+	int			ram;
+	int			not_ram;
+};
+
+static int pagerange_is_ram_callback(unsigned long initial_pfn,
+				unsigned long total_nr_pages, void *arg)
+{
+	struct pagerange_is_ram_state *state = arg;
+
+	state->not_ram |= initial_pfn > state->cur_pfn;
+	state->ram |= total_nr_pages > 0;
+	state->cur_pfn = initial_pfn + total_nr_pages;
+
+	return state->ram && state->not_ram;
+}
+
 static int pat_pagerange_is_ram(resource_size_t start, resource_size_t end)
 {
-	int ram_page = 0, not_rampage = 0;
-	unsigned long page_nr;
+	int ret = 0;
+	unsigned long start_pfn = start >> PAGE_SHIFT;
+	unsigned long end_pfn = (end + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	struct pagerange_is_ram_state state = {start_pfn, 0, 0};
 
-	for (page_nr = (start >> PAGE_SHIFT); page_nr < (end >> PAGE_SHIFT);
-	     ++page_nr) {
-		/*
-		 * For legacy reasons, physical address range in the legacy ISA
-		 * region is tracked as non-RAM. This will allow users of
-		 * /dev/mem to map portions of legacy ISA region, even when
-		 * some of those portions are listed(or not even listed) with
-		 * different e820 types(RAM/reserved/..)
-		 */
-		if (page_nr >= (ISA_END_ADDRESS >> PAGE_SHIFT) &&
-		    page_is_ram(page_nr))
-			ram_page = 1;
-		else
-			not_rampage = 1;
-
-		if (ram_page == not_rampage)
-			return -1;
-	}
+	/*
+	 * For legacy reasons, physical address range in the legacy ISA
+	 * region is tracked as non-RAM. This will allow users of
+	 * /dev/mem to map portions of legacy ISA region, even when
+	 * some of those portions are listed(or not even listed) with
+	 * different e820 types(RAM/reserved/..)
+	 */
+	if (start_pfn < ISA_END_ADDRESS >> PAGE_SHIFT)
+		start_pfn = ISA_END_ADDRESS >> PAGE_SHIFT;
 
-	return ram_page;
+	if (start_pfn < end_pfn)
+		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
+				&state, pagerange_is_ram_callback);
+	return (ret > 0) ? -1 : (state.ram ? 1 : 0);
 }
 
 /*
-- 
1.7.0.4