[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, 3 insertions, 3 deletions

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 88af42f5e04a..c937d6e65502 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -126,8 +126,8 @@ BUFFER_FNS(Eopnotsupp, eopnotsupp)
 /* If we *know* page->private refers to buffer_heads */
 #define page_buffers(page)                                      \
         ({                                                      \
-                BUG_ON(!PagePrivate(page));             \
-                ((struct buffer_head *)(page)->private);        \
+                BUG_ON(!PagePrivate(page));                     \
+                ((struct buffer_head *)page_private(page));     \
         })
 #define page_has_buffers(page)  PagePrivate(page)
 
@@ -219,7 +219,7 @@ static inline void attach_page_buffers(struct page *page,
 {
         page_cache_get(page);
         SetPagePrivate(page);
-        page->private = (unsigned long)head;
+        set_page_private(page, (unsigned long)head);
 }
 
 static inline void get_bh(struct buffer_head *bh)