[PATCH 34/62] page cache: Use xarray for adding pages

Matthew Wilcox
2017-11-22T21:14:36Z

Use the XArray APIs to add and replace pages in the page cache. This
removes two uses of the radix tree preload API and is significantly
shorter code.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
---
include/linux/xarray.h | 13 +++++
mm/filemap.c | 131 +++++++++++++++++++++----------------------------
2 files changed, 69 insertions(+), 75 deletions(-)

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 8d7cd70beb8e..1648eda4a20d 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -542,6 +542,19 @@ static inline void xas_set_order(struct xa_state *xas, unsigned long index,
xas->xa_node = XAS_RESTART;
}

+/**
+ * xas_set_update() - Set up XArray operation state for a callback.
+ * @xas: XArray operation state.
+ * @update: Function to call when updating a node.
+ *
+ * The XArray can notify a caller after it has updated an xa_node.
+ * This is advanced functionality and is only needed by the page cache.
+ */
+static inline void xas_set_update(struct xa_state *xas, xa_update_node_t update)
+{
+ xas->xa_update = update;
+}
+
/* Skip over any of these entries when iterating */
static inline bool xa_iter_skip(void *entry)
{
diff --git a/mm/filemap.c b/mm/filemap.c
index accc350f9544..1d520748789b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -112,34 +112,6 @@
* ->tasklist_lock (memory_failure, collect_procs_ao)
*/

-static int page_cache_tree_insert(struct address_space *mapping,
- struct page *page, void **shadowp)
-{
- struct radix_tree_node *node;
- void **slot;
- int error;
-
- error = __radix_tree_create(&mapping->pages, page->index, 0,
- &node, &slot);
- if (error)
- return error;
- if (*slot) {
- void *p;
-
- p = radix_tree_deref_slot_protected(slot, &mapping->pages.xa_lock);
- if (!xa_is_value(p))
- return -EEXIST;
-
- mapping->nrexceptional--;
- if (shadowp)
- *shadowp = p;
- }
- __radix_tree_replace(&mapping->pages, node, slot, page,
- workingset_lookup_update(mapping));
- mapping->nrpages++;
- return 0;
-}
-
static void page_cache_tree_delete(struct address_space *mapping,
struct page *page, void *shadow)
{
@@ -775,51 +747,44 @@ EXPORT_SYMBOL(file_write_and_wait_range);
* locked. This function does not add the new page to the LRU, the
* caller must do that.
*
- * The remove + add is atomic. The only way this function can fail is
- * memory allocation failure.
+ * The remove + add is atomic. This function cannot fail.
*/
int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
{
- int error;
+ XA_STATE(xas, old->index);
+ struct address_space *mapping = old->mapping;
+ void (*freepage)(struct page *) = mapping->a_ops->freepage;
+ pgoff_t offset = old->index;
+ unsigned long flags;

VM_BUG_ON_PAGE(!PageLocked(old), old);
VM_BUG_ON_PAGE(!PageLocked(new), new);
VM_BUG_ON_PAGE(new->mapping, new);

- error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
- if (!error) {
- struct address_space *mapping = old->mapping;
- void (*freepage)(struct page *);
- unsigned long flags;
-
- pgoff_t offset = old->index;
- freepage = mapping->a_ops->freepage;
-
- get_page(new);
- new->mapping = mapping;
- new->index = offset;
+ get_page(new);
+ new->mapping = mapping;
+ new->index = offset;

- xa_lock_irqsave(&mapping->pages, flags);
- __delete_from_page_cache(old, NULL);
- error = page_cache_tree_insert(mapping, new, NULL);
- BUG_ON(error);
+ xa_lock_irqsave(&mapping->pages, flags);
+ xas_store(&mapping->pages, &xas, new);

- /*
- * hugetlb pages do not participate in page cache accounting.
- */
- if (!PageHuge(new))
- __inc_node_page_state(new, NR_FILE_PAGES);
- if (PageSwapBacked(new))
- __inc_node_page_state(new, NR_SHMEM);
- xa_unlock_irqrestore(&mapping->pages, flags);
- mem_cgroup_migrate(old, new);
- radix_tree_preload_end();
- if (freepage)
- freepage(old);
- put_page(old);
- }
+ old->mapping = NULL;
+ /* hugetlb pages do not participate in page cache accounting. */
+ if (!PageHuge(old))
+ __dec_node_page_state(new, NR_FILE_PAGES);
+ if (!PageHuge(new))
+ __inc_node_page_state(new, NR_FILE_PAGES);
+ if (PageSwapBacked(old))
+ __dec_node_page_state(new, NR_SHMEM);
+ if (PageSwapBacked(new))
+ __inc_node_page_state(new, NR_SHMEM);
+ xa_unlock_irqrestore(&mapping->pages, flags);
+ mem_cgroup_migrate(old, new);
+ if (freepage)
+ freepage(old);
+ put_page(old);

- return error;
+ return 0;
}
EXPORT_SYMBOL_GPL(replace_page_cache_page);

@@ -828,12 +793,15 @@ static int __add_to_page_cache_locked(struct page *page,
pgoff_t offset, gfp_t gfp_mask,
void **shadowp)
{
+ XA_STATE(xas, offset);
int huge = PageHuge(page);
struct mem_cgroup *memcg;
int error;
+ void *old;

VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(PageSwapBacked(page), page);
+ xas_set_update(&xas, workingset_lookup_update(mapping));

if (!huge) {
error = mem_cgroup_try_charge(page, current->mm,
@@ -842,35 +810,48 @@ static int __add_to_page_cache_locked(struct page *page,
return error;
}

- error = radix_tree_maybe_preload(gfp_mask & ~__GFP_HIGHMEM);
- if (error) {
- if (!huge)
- mem_cgroup_cancel_charge(page, memcg, false);
- return error;
- }
-
get_page(page);
page->mapping = mapping;
page->index = offset;

+retry:
xa_lock_irq(&mapping->pages);
- error = page_cache_tree_insert(mapping, page, shadowp);
- radix_tree_preload_end();
- if (unlikely(error))
- goto err_insert;
+ old = xas_create(&mapping->pages, &xas);
+ error = xas_error(&xas);
+ if (error) {
+ xa_unlock_irq(&mapping->pages);
+ if (xas_nomem(&xas, gfp_mask & ~__GFP_HIGHMEM))
+ goto retry;
+ goto error;
+ }
+
+ if (xa_is_value(old)) {
+ mapping->nrexceptional--;
+ if (shadowp)
+ *shadowp = old;
+ } else if (old) {
+ goto exist;
+ }
+
+ xas_store(&mapping->pages, &xas, page);
+ mapping->nrpages++;

/* hugetlb pages do not participate in page cache accounting. */
if (!huge)
__inc_node_page_state(page, NR_FILE_PAGES);
xa_unlock_irq(&mapping->pages);
+ xas_destroy(&xas);
if (!huge)
mem_cgroup_commit_charge(page, memcg, false, false);
trace_mm_filemap_add_to_page_cache(page);
return 0;
-err_insert:
+exist:
+ xa_unlock_irq(&mapping->pages);
+ error = -EEXIST;
+error:
+ xas_destroy(&xas);
page->mapping = NULL;
/* Leave page->index set: truncation relies upon it */
- xa_unlock_irq(&mapping->pages);
if (!huge)
mem_cgroup_cancel_charge(page, memcg, false);
put_page(page);
--
2.15.0