Dave Hansen wrote:
> On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <[email protected]>
> >
> > For tail page we call __get_page_tail(). It has the same semantics, but
> > for tail page.
>
> page_cache_get_speculative() has a ~50-line comment above it with lots
> of scariness about grace periods and RCU. A two line comment saying
> that the semantics are the same doesn't make me feel great that you've
> done your homework here.
Okay. Will fix commit message and the comment.
> Are there any performance implications here? __get_page_tail() says:
> "It implements the slow path of get_page().".
> page_cache_get_speculative() seems awfully speculative which would make
> me think that it is part of a _fast_ path.
It's slow path in the sense that we have to do more for tail page then for
non-compound or head page.
Probably, we can get it a bit faster by unrolling function calls and doing
only what is relevant for our case. Like this:
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index ad60dcc..57ad1ae 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -161,6 +161,8 @@ void release_pages(struct page **pages, int nr, int cold);
*/
static inline int page_cache_get_speculative(struct page *page)
{
+ struct page *page_head = compound_trans_head(page);
+
VM_BUG_ON(in_interrupt());
#ifdef CONFIG_TINY_RCU
@@ -176,11 +178,11 @@ static inline int page_cache_get_speculative(struct page
*page)
* disabling preempt, and hence no need for the "speculative get" that
* SMP requires.
*/
- VM_BUG_ON(page_count(page) == 0);
+ VM_BUG_ON(page_count(page_head) == 0);
atomic_inc(&page->_count);
#else
- if (unlikely(!get_page_unless_zero(page))) {
+ if (unlikely(!get_page_unless_zero(page_head))) {
/*
* Either the page has been freed, or will be freed.
* In either case, retry here and the caller should
@@ -189,7 +191,23 @@ static inline int page_cache_get_speculative(struct page
*page)
return 0;
}
#endif
- VM_BUG_ON(PageTail(page));
+
+ if (unlikely(PageTransTail(page))) {
+ unsigned long flags;
+ int got = 0;
+
+ flags = compound_lock_irqsave(page_head);
+ if (likely(PageTransTail(page))) {
+ atomic_inc(&page->_mapcount);
+ got = 1;
+ }
+ compound_unlock_irqrestore(page_head, flags);
+
+ if (unlikely(!got))
+ put_page(page_head);
+
+ return got;
+ }
return 1;
}
What do you think? Is it better?
--
Kirill A. Shutemov
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