[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, 1 insertions, 1 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 8aa344e88489..f560b41c8f61 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -152,7 +152,7 @@ static int sync_page(void *word)
 	 * in the ->sync_page() methods make essential use of the
 	 * page_mapping(), merely passing the page down to the backing
 	 * device's unplug functions when it's non-NULL, which in turn
-	 * ignore it for all cases but swap, where only page->private is
+	 * ignore it for all cases but swap, where only page_private(page) is
 	 * of interest. When page_mapping() does go NULL, the entire
 	 * call stack gracefully ignores the page and returns.
 	 * -- wli