[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, 4 insertions, 4 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 132164f7d0a7..cafc1edcbeba 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -83,7 +83,7 @@ static int __add_to_swap_cache(struct page *page, swp_entry_t entry,
                         page_cache_get(page);
                         SetPageLocked(page);
                         SetPageSwapCache(page);
-                        page->private = entry.val;
+                        set_page_private(page, entry.val);
                         total_swapcache_pages++;
                         pagecache_acct(1);
                 }
@@ -126,8 +126,8 @@ void __delete_from_swap_cache(struct page *page)
         BUG_ON(PageWriteback(page));
         BUG_ON(PagePrivate(page));
 
-        radix_tree_delete(&swapper_space.page_tree, page->private);
-        page->private = 0;
+        radix_tree_delete(&swapper_space.page_tree, page_private(page));
+        set_page_private(page, 0);
         ClearPageSwapCache(page);
         total_swapcache_pages--;
         pagecache_acct(-1);
@@ -197,7 +197,7 @@ void delete_from_swap_cache(struct page *page)
 {
         swp_entry_t entry;
 
-        entry.val = page->private;
+        entry.val = page_private(page);
 
         write_lock_irq(&swapper_space.tree_lock);
         __delete_from_swap_cache(page);