tmpfs: convert shmem_unuse_inode to radix-swap

Convert shmem_unuse_inode() to use a lockless gang lookup of the radix
tree, searching for matching swap.

This is somewhat slower than the old method: because of repeated radix
tree descents, because of copying entries up, but probably most because
the old method noted and skipped once a vector page was cleared of swap.
Perhaps we can devise a use of radix tree tagging to achieve that later.

shmem_add_to_page_cache() uses shmem_radix_tree_replace() to compensate
for the lockless lookup by checking that the expected entry is in place,
under lock.  It is not very satisfactory to be copying this much from
add_to_page_cache_locked(), but I think easier to sell than insisting
that every caller of add_to_page_cache*() go through the extras.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 107 insertions, 26 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 4439b7d55819..174f97188e8a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -262,6 +262,55 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
 }
 
 /*
+ * Like add_to_page_cache_locked, but error if expected item has gone.
+ */
+static int shmem_add_to_page_cache(struct page *page,
+                                   struct address_space *mapping,
+                                   pgoff_t index, gfp_t gfp, void *expected)
+{
+        int error;
+
+        VM_BUG_ON(!PageLocked(page));
+        VM_BUG_ON(!PageSwapBacked(page));
+
+        error = mem_cgroup_cache_charge(page, current->mm,
+                                                gfp & GFP_RECLAIM_MASK);
+        if (error)
+                goto out;
+        if (!expected)
+                error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);
+        if (!error) {
+                page_cache_get(page);
+                page->mapping = mapping;
+                page->index = index;
+
+                spin_lock_irq(&mapping->tree_lock);
+                if (!expected)
+                        error = radix_tree_insert(&mapping->page_tree,
+                                                        index, page);
+                else
+                        error = shmem_radix_tree_replace(mapping, index,
+                                                        expected, page);
+                if (!error) {
+                        mapping->nrpages++;
+                        __inc_zone_page_state(page, NR_FILE_PAGES);
+                        __inc_zone_page_state(page, NR_SHMEM);
+                        spin_unlock_irq(&mapping->tree_lock);
+                } else {
+                        page->mapping = NULL;
+                        spin_unlock_irq(&mapping->tree_lock);
+                        page_cache_release(page);
+                }
+                if (!expected)
+                        radix_tree_preload_end();
+        }
+        if (error)
+                mem_cgroup_uncharge_cache_page(page);
+out:
+        return error;
+}
+
+/*
  * Like find_get_pages, but collecting swap entries as well as pages.
  */
 static unsigned shmem_find_get_pages_and_swap(struct address_space *mapping,
@@ -309,6 +358,42 @@ export:
 }
 
 /*
+ * Lockless lookup of swap entry in radix tree, avoiding refcount on pages.
+ */
+static pgoff_t shmem_find_swap(struct address_space *mapping, void *radswap)
+{
+        void  **slots[PAGEVEC_SIZE];
+        pgoff_t indices[PAGEVEC_SIZE];
+        unsigned int nr_found;
+
+restart:
+        nr_found = 1;
+        indices[0] = -1;
+        while (nr_found) {
+                pgoff_t index = indices[nr_found - 1] + 1;
+                unsigned int i;
+
+                rcu_read_lock();
+                nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
+                                        slots, indices, index, PAGEVEC_SIZE);
+                for (i = 0; i < nr_found; i++) {
+                        void *item = radix_tree_deref_slot(slots[i]);
+                        if (radix_tree_deref_retry(item)) {
+                                rcu_read_unlock();
+                                goto restart;
+                        }
+                        if (item == radswap) {
+                                rcu_read_unlock();
+                                return indices[i];
+                        }
+                }
+                rcu_read_unlock();
+                cond_resched();
+        }
+        return -1;
+}
+
+/*
  * Remove swap entry from radix tree, free the swap and its page cache.
  */
 static int shmem_free_swap(struct address_space *mapping,
@@ -515,23 +600,21 @@ static void shmem_evict_inode(struct inode *inode)
         end_writeback(inode);
 }
 
+/*
+ * If swap found in inode, free it and move page from swapcache to filecache.
+ */
 static int shmem_unuse_inode(struct shmem_inode_info *info,
                              swp_entry_t swap, struct page *page)
 {
         struct address_space *mapping = info->vfs_inode.i_mapping;
+        void *radswap;
         pgoff_t index;
         int error;
 
-        for (index = 0; index < SHMEM_NR_DIRECT; index++)
-                if (shmem_get_swap(info, index).val == swap.val)
-                        goto found;
-        return 0;
-found:
-        spin_lock(&info->lock);
-        if (shmem_get_swap(info, index).val != swap.val) {
-                spin_unlock(&info->lock);
+        radswap = swp_to_radix_entry(swap);
+        index = shmem_find_swap(mapping, radswap);
+        if (index == -1)
                 return 0;
-        }
 
         /*
          * Move _head_ to start search for next from here.
@@ -547,23 +630,30 @@ found:
          * but also to hold up shmem_evict_inode(): so inode cannot be freed
          * beneath us (pagelock doesn't help until the page is in pagecache).
          */
-        error = add_to_page_cache_locked(page, mapping, index, GFP_NOWAIT);
+        error = shmem_add_to_page_cache(page, mapping, index,
+                                                GFP_NOWAIT, radswap);
         /* which does mem_cgroup_uncharge_cache_page on error */
 
         if (error != -ENOMEM) {
+                /*
+                 * Truncation and eviction use free_swap_and_cache(), which
+                 * only does trylock page: if we raced, best clean up here.
+                 */
                 delete_from_swap_cache(page);
                 set_page_dirty(page);
-                shmem_put_swap(info, index, (swp_entry_t){0});
-                info->swapped--;
-                swap_free(swap);
+                if (!error) {
+                        spin_lock(&info->lock);
+                        info->swapped--;
+                        spin_unlock(&info->lock);
+                        swap_free(swap);
+                }
                 error = 1;      /* not an error, but entry was found */
         }
-        spin_unlock(&info->lock);
         return error;
 }
 
 /*
- * shmem_unuse() search for an eventually swapped out shmem page.
+ * Search through swapped inodes to find and replace swap by page.
  */
 int shmem_unuse(swp_entry_t swap, struct page *page)
 {
@@ -576,20 +666,12 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
          * Charge page using GFP_KERNEL while we can wait, before taking
          * the shmem_swaplist_mutex which might hold up shmem_writepage().
          * Charged back to the user (not to caller) when swap account is used.
-         * add_to_page_cache() will be called with GFP_NOWAIT.
+         * shmem_add_to_page_cache() will be called with GFP_NOWAIT.
          */
         error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);
         if (error)
                 goto out;
-        /*
-         * Try to preload while we can wait, to not make a habit of
-         * draining atomic reserves; but don't latch on to this cpu,
-         * it's okay if sometimes we get rescheduled after this.
-         */
-        error = radix_tree_preload(GFP_KERNEL);
-        if (error)
-                goto uncharge;
-        radix_tree_preload_end();
+        /* No radix_tree_preload: swap entry keeps a place for page in tree */
 
         mutex_lock(&shmem_swaplist_mutex);
         list_for_each_safe(this, next, &shmem_swaplist) {
@@ -608,7 +690,6 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
         }
         mutex_unlock(&shmem_swaplist_mutex);
 
-uncharge:
         if (!found)
                 mem_cgroup_uncharge_cache_page(page);
         if (found < 0)