[PATCH] mm: split page table lock

Christoph Lameter demonstrated very poor scalability on the SGI 512-way, with
a many-threaded application which concurrently initializes different parts of
a large anonymous area.

This patch corrects that, by using a separate spinlock per page table page, to
guard the page table entries in that page, instead of using the mm's single
page_table_lock.  (But even then, page_table_lock is still used to guard page
table allocation, and anon_vma allocation.)

In this implementation, the spinlock is tucked inside the struct page of the
page table page: with a BUILD_BUG_ON in case it overflows - which it would in
the case of 32-bit PA-RISC with spinlock debugging enabled.

Splitting the lock is not quite for free: another cacheline access.  Ideally,
I suppose we would use split ptlock only for multi-threaded processes on
multi-cpu machines; but deciding that dynamically would have its own costs.
So for now enable it by config, at some number of cpus - since the Kconfig
language doesn't support inequalities, let preprocessor compare that with
NR_CPUS.  But I don't think it's worth being user-configurable: for good
testing of both split and unsplit configs, split now at 4 cpus, and perhaps
change that to 8 later.

There is a benefit even for singly threaded processes: kswapd can be attacking
one part of the mm while another part is busy faulting.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 12 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 37777f4c11f8..dc25565a61e9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -71,9 +71,6 @@
 /* Pretend that each entry is of this size in directory's i_size */
 #define BOGO_DIRENT_SIZE 20
 
-/* Keep swapped page count in private field of indirect struct page */
-#define nr_swapped		private
-
 /* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
 enum sgp_type {
 	SGP_QUICK,	/* don't try more than file page cache lookup */
@@ -324,8 +321,10 @@ static void shmem_swp_set(struct shmem_inode_info *info, swp_entry_t *entry, uns
 
 	entry->val = value;
 	info->swapped += incdec;
-	if ((unsigned long)(entry - info->i_direct) >= SHMEM_NR_DIRECT)
-		kmap_atomic_to_page(entry)->nr_swapped += incdec;
+	if ((unsigned long)(entry - info->i_direct) >= SHMEM_NR_DIRECT) {
+		struct page *page = kmap_atomic_to_page(entry);
+		set_page_private(page, page_private(page) + incdec);
+	}
 }
 
 /*
@@ -368,9 +367,8 @@ static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long
 
 		spin_unlock(&info->lock);
 		page = shmem_dir_alloc(mapping_gfp_mask(inode->i_mapping) | __GFP_ZERO);
-		if (page) {
-			page->nr_swapped = 0;
-		}
+		if (page)
+			set_page_private(page, 0);
 		spin_lock(&info->lock);
 
 		if (!page) {
@@ -561,7 +559,7 @@ static void shmem_truncate(struct inode *inode)
 			diroff = 0;
 		}
 		subdir = dir[diroff];
-		if (subdir && subdir->nr_swapped) {
+		if (subdir && page_private(subdir)) {
 			size = limit - idx;
 			if (size > ENTRIES_PER_PAGE)
 				size = ENTRIES_PER_PAGE;
@@ -572,10 +570,10 @@ static void shmem_truncate(struct inode *inode)
 			nr_swaps_freed += freed;
 			if (offset)
 				spin_lock(&info->lock);
-			subdir->nr_swapped -= freed;
+			set_page_private(subdir, page_private(subdir) - freed);
 			if (offset)
 				spin_unlock(&info->lock);
-			BUG_ON(subdir->nr_swapped > offset);
+			BUG_ON(page_private(subdir) > offset);
 		}
 		if (offset)
 			offset = 0;
@@ -743,7 +741,7 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
 			dir = shmem_dir_map(subdir);
 		}
 		subdir = *dir;
-		if (subdir && subdir->nr_swapped) {
+		if (subdir && page_private(subdir)) {
 			ptr = shmem_swp_map(subdir);
 			size = limit - idx;
 			if (size > ENTRIES_PER_PAGE)