mm: lockless pagecache
Combine page_cache_get_speculative with lockless radix tree lookups to
introduce lockless page cache lookups (ie. no mapping->tree_lock on the
read-side).
The only atomicity changes this introduces is that the gang pagecache
lookup functions now behave as if they are implemented with multiple
find_get_page calls, rather than operating on a snapshot of the pages. In
practice, this atomicity guarantee is not used anyway, and it is to
replace individual lookups, so these semantics are natural.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/filemap.c b/mm/filemap.c
index 4e182a9..feb8448 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -637,15 +637,35 @@
* Is there a pagecache struct page at the given (mapping, offset) tuple?
* If yes, increment its refcount and return it; if no, return NULL.
*/
-struct page * find_get_page(struct address_space *mapping, pgoff_t offset)
+struct page *find_get_page(struct address_space *mapping, pgoff_t offset)
{
+ void **pagep;
struct page *page;
- read_lock_irq(&mapping->tree_lock);
- page = radix_tree_lookup(&mapping->page_tree, offset);
- if (page)
- page_cache_get(page);
- read_unlock_irq(&mapping->tree_lock);
+ rcu_read_lock();
+repeat:
+ page = NULL;
+ pagep = radix_tree_lookup_slot(&mapping->page_tree, offset);
+ if (pagep) {
+ page = radix_tree_deref_slot(pagep);
+ if (unlikely(!page || page == RADIX_TREE_RETRY))
+ goto repeat;
+
+ if (!page_cache_get_speculative(page))
+ goto repeat;
+
+ /*
+ * Has the page moved?
+ * This is part of the lockless pagecache protocol. See
+ * include/linux/pagemap.h for details.
+ */
+ if (unlikely(page != *pagep)) {
+ page_cache_release(page);
+ goto repeat;
+ }
+ }
+ rcu_read_unlock();
+
return page;
}
EXPORT_SYMBOL(find_get_page);
@@ -660,32 +680,22 @@
*
* Returns zero if the page was not present. find_lock_page() may sleep.
*/
-struct page *find_lock_page(struct address_space *mapping,
- pgoff_t offset)
+struct page *find_lock_page(struct address_space *mapping, pgoff_t offset)
{
struct page *page;
repeat:
- read_lock_irq(&mapping->tree_lock);
- page = radix_tree_lookup(&mapping->page_tree, offset);
+ page = find_get_page(mapping, offset);
if (page) {
- page_cache_get(page);
- if (TestSetPageLocked(page)) {
- read_unlock_irq(&mapping->tree_lock);
- __lock_page(page);
-
- /* Has the page been truncated while we slept? */
- if (unlikely(page->mapping != mapping)) {
- unlock_page(page);
- page_cache_release(page);
- goto repeat;
- }
- VM_BUG_ON(page->index != offset);
- goto out;
+ lock_page(page);
+ /* Has the page been truncated? */
+ if (unlikely(page->mapping != mapping)) {
+ unlock_page(page);
+ page_cache_release(page);
+ goto repeat;
}
+ VM_BUG_ON(page->index != offset);
}
- read_unlock_irq(&mapping->tree_lock);
-out:
return page;
}
EXPORT_SYMBOL(find_lock_page);
@@ -751,13 +761,39 @@
{
unsigned int i;
unsigned int ret;
+ unsigned int nr_found;
- read_lock_irq(&mapping->tree_lock);
- ret = radix_tree_gang_lookup(&mapping->page_tree,
- (void **)pages, start, nr_pages);
- for (i = 0; i < ret; i++)
- page_cache_get(pages[i]);
- read_unlock_irq(&mapping->tree_lock);
+ rcu_read_lock();
+restart:
+ nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
+ (void ***)pages, start, nr_pages);
+ ret = 0;
+ for (i = 0; i < nr_found; i++) {
+ struct page *page;
+repeat:
+ page = radix_tree_deref_slot((void **)pages[i]);
+ if (unlikely(!page))
+ continue;
+ /*
+ * this can only trigger if nr_found == 1, making livelock
+ * a non issue.
+ */
+ if (unlikely(page == RADIX_TREE_RETRY))
+ goto restart;
+
+ if (!page_cache_get_speculative(page))
+ goto repeat;
+
+ /* Has the page moved? */
+ if (unlikely(page != *((void **)pages[i]))) {
+ page_cache_release(page);
+ goto repeat;
+ }
+
+ pages[ret] = page;
+ ret++;
+ }
+ rcu_read_unlock();
return ret;
}
@@ -778,19 +814,44 @@
{
unsigned int i;
unsigned int ret;
+ unsigned int nr_found;
- read_lock_irq(&mapping->tree_lock);
- ret = radix_tree_gang_lookup(&mapping->page_tree,
- (void **)pages, index, nr_pages);
- for (i = 0; i < ret; i++) {
- if (pages[i]->mapping == NULL || pages[i]->index != index)
+ rcu_read_lock();
+restart:
+ nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
+ (void ***)pages, index, nr_pages);
+ ret = 0;
+ for (i = 0; i < nr_found; i++) {
+ struct page *page;
+repeat:
+ page = radix_tree_deref_slot((void **)pages[i]);
+ if (unlikely(!page))
+ continue;
+ /*
+ * this can only trigger if nr_found == 1, making livelock
+ * a non issue.
+ */
+ if (unlikely(page == RADIX_TREE_RETRY))
+ goto restart;
+
+ if (page->mapping == NULL || page->index != index)
break;
- page_cache_get(pages[i]);
+ if (!page_cache_get_speculative(page))
+ goto repeat;
+
+ /* Has the page moved? */
+ if (unlikely(page != *((void **)pages[i]))) {
+ page_cache_release(page);
+ goto repeat;
+ }
+
+ pages[ret] = page;
+ ret++;
index++;
}
- read_unlock_irq(&mapping->tree_lock);
- return i;
+ rcu_read_unlock();
+ return ret;
}
EXPORT_SYMBOL(find_get_pages_contig);
@@ -810,15 +871,43 @@
{
unsigned int i;
unsigned int ret;
+ unsigned int nr_found;
- read_lock_irq(&mapping->tree_lock);
- ret = radix_tree_gang_lookup_tag(&mapping->page_tree,
- (void **)pages, *index, nr_pages, tag);
- for (i = 0; i < ret; i++)
- page_cache_get(pages[i]);
+ rcu_read_lock();
+restart:
+ nr_found = radix_tree_gang_lookup_tag_slot(&mapping->page_tree,
+ (void ***)pages, *index, nr_pages, tag);
+ ret = 0;
+ for (i = 0; i < nr_found; i++) {
+ struct page *page;
+repeat:
+ page = radix_tree_deref_slot((void **)pages[i]);
+ if (unlikely(!page))
+ continue;
+ /*
+ * this can only trigger if nr_found == 1, making livelock
+ * a non issue.
+ */
+ if (unlikely(page == RADIX_TREE_RETRY))
+ goto restart;
+
+ if (!page_cache_get_speculative(page))
+ goto repeat;
+
+ /* Has the page moved? */
+ if (unlikely(page != *((void **)pages[i]))) {
+ page_cache_release(page);
+ goto repeat;
+ }
+
+ pages[ret] = page;
+ ret++;
+ }
+ rcu_read_unlock();
+
if (ret)
*index = pages[ret - 1]->index + 1;
- read_unlock_irq(&mapping->tree_lock);
+
return ret;
}
EXPORT_SYMBOL(find_get_pages_tag);