[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>

4 files changed, 13 insertions, 12 deletions

diff --git a/fs/afs/file.c b/fs/afs/file.c
index 0d57698..4975c9c 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -291,8 +291,8 @@ static int afs_file_releasepage(struct page *page, gfp_t gfp_flags)
 		cachefs_uncache_page(vnode->cache, page);
 #endif
 
-		pageio = (struct cachefs_page *) page->private;
-		page->private = 0;
+		pageio = (struct cachefs_page *) page_private(page);
+		set_page_private(page, 0);
 		ClearPagePrivate(page);
 
 		if (pageio)
diff --git a/fs/buffer.c b/fs/buffer.c
index b166798..2066e4c 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -96,7 +96,7 @@ static void
 __clear_page_buffers(struct page *page)
 {
 	ClearPagePrivate(page);
-	page->private = 0;
+	set_page_private(page, 0);
 	page_cache_release(page);
 }
 
diff --git a/fs/jfs/jfs_metapage.c b/fs/jfs/jfs_metapage.c
index 26091a5..8a53981 100644
--- a/fs/jfs/jfs_metapage.c
+++ b/fs/jfs/jfs_metapage.c
@@ -86,7 +86,7 @@ struct meta_anchor {
 	atomic_t io_count;
 	struct metapage *mp[MPS_PER_PAGE];
 };
-#define mp_anchor(page) ((struct meta_anchor *)page->private)
+#define mp_anchor(page) ((struct meta_anchor *)page_private(page))
 
 static inline struct metapage *page_to_mp(struct page *page, uint offset)
 {
@@ -108,7 +108,7 @@ static inline int insert_metapage(struct page *page, struct metapage *mp)
 		if (!a)
 			return -ENOMEM;
 		memset(a, 0, sizeof(struct meta_anchor));
-		page->private = (unsigned long)a;
+		set_page_private(page, (unsigned long)a);
 		SetPagePrivate(page);
 		kmap(page);
 	}
@@ -136,7 +136,7 @@ static inline void remove_metapage(struct page *page, struct metapage *mp)
 	a->mp[index] = NULL;
 	if (--a->mp_count == 0) {
 		kfree(a);
-		page->private = 0;
+		set_page_private(page, 0);
 		ClearPagePrivate(page);
 		kunmap(page);
 	}
@@ -156,13 +156,13 @@ static inline void dec_io(struct page *page, void (*handler) (struct page *))
 #else
 static inline struct metapage *page_to_mp(struct page *page, uint offset)
 {
-	return PagePrivate(page) ? (struct metapage *)page->private : NULL;
+	return PagePrivate(page) ? (struct metapage *)page_private(page) : NULL;
 }
 
 static inline int insert_metapage(struct page *page, struct metapage *mp)
 {
 	if (mp) {
-		page->private = (unsigned long)mp;
+		set_page_private(page, (unsigned long)mp);
 		SetPagePrivate(page);
 		kmap(page);
 	}
@@ -171,7 +171,7 @@ static inline int insert_metapage(struct page *page, struct metapage *mp)
 
 static inline void remove_metapage(struct page *page, struct metapage *mp)
 {
-	page->private = 0;
+	set_page_private(page, 0);
 	ClearPagePrivate(page);
 	kunmap(page);
 }
diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index ba4767c..4cd46ab 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -181,8 +181,9 @@ set_page_region(
 	size_t		offset,
 	size_t		length)
 {
-	page->private |= page_region_mask(offset, length);
-	if (page->private == ~0UL)
+	set_page_private(page,
+		page_private(page) | page_region_mask(offset, length));
+	if (page_private(page) == ~0UL)
 		SetPageUptodate(page);
 }
 
@@ -194,7 +195,7 @@ test_page_region(
 {
 	unsigned long	mask = page_region_mask(offset, length);
 
-	return (mask && (page->private & mask) == mask);
+	return (mask && (page_private(page) & mask) == mask);
 }
 
 /*