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SubjectRe: 2.6.22 -mm merge plans -- vm bugfixes
Hugh Dickins wrote:
> On Thu, 3 May 2007, Nick Piggin wrote:
>
>>The problem is that lock/unlock_page is expensive on powerpc, and
>>if we improve that, we improve more than just the fault handler...
>>
>>The attached patch gets performance up a bit by avoiding some
>>barriers and some cachelines:
>
>
> There's a strong whiff of raciness about this...
> but I could very easily be wrong.
>
>
>>Index: linux-2.6/mm/filemap.c
>>===================================================================
>>--- linux-2.6.orig/mm/filemap.c 2007-05-02 15:00:26.000000000 +1000
>>+++ linux-2.6/mm/filemap.c 2007-05-03 08:34:32.000000000 +1000
>>@@ -532,11 +532,13 @@
>> */
>> void fastcall unlock_page(struct page *page)
>> {
>>+ VM_BUG_ON(!PageLocked(page));
>> smp_mb__before_clear_bit();
>>- if (!TestClearPageLocked(page))
>>- BUG();
>>- smp_mb__after_clear_bit();
>>- wake_up_page(page, PG_locked);
>>+ ClearPageLocked(page);
>>+ if (unlikely(test_bit(PG_waiters, &page->flags))) {
>>+ clear_bit(PG_waiters, &page->flags);
>>+ wake_up_page(page, PG_locked);
>>+ }
>> }
>> EXPORT_SYMBOL(unlock_page);
>>
>>@@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!)
>> {
>> DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
>>
>>+ set_bit(PG_waiters, &page->flags);
>>+ if (unlikely(!TestSetPageLocked(page))) {
>
>
> What happens if another cpu is coming through __lock_page at the
> same time, did its set_bit, now finds PageLocked, and so proceeds
> to the __wait_on_bit_lock? But this cpu now clears PG_waiters,
> so this task's unlock_page won't wake the other?

You're right, we can't clear the bit here. Doubt it mattered much anyway?

BTW. I also forgot an smp_mb__after_clear_bit() before the wake_up_page
above... that barrier is in the slow path as well though, so it shouldn't
matter either.

>
>
>>+ clear_bit(PG_waiters, &page->flags);
>>+ return;
>>+ }
>> __wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
>> TASK_UNINTERRUPTIBLE);
>> }
>
>


--
SUSE Labs, Novell Inc.
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