mm: batch radix tree operations when truncating pages

Currently we remove pages from the radix tree one by one.  To speed up
page cache truncation, lock several pages at once and free them in one
go.  This allows us to batch radix tree operations in a more efficient
way and also save round-trips on mapping->tree_lock.  As a result we
gain about 20% speed improvement in page cache truncation.

Data from a simple benchmark timing 10000 truncates of 1024 pages (on
ext4 on ramdisk but the filesystem is barely visible in the profiles).
The range shows 1% and 95% percentiles of the measured times:

  4.14-rc2	4.14-rc2 + batched truncation
  248-256	209-219
  249-258	209-217
  248-255	211-239
  248-255	209-217
  247-256	210-218

[jack@suse.cz: convert delete_from_page_cache_batch() to pagevec]
  Link: http://lkml.kernel.org/r/20171018111648.13714-1-jack@suse.cz
[akpm@linux-foundation.org: move struct pagevec forward declaration to top-of-file]
Link: http://lkml.kernel.org/r/20171010151937.26984-8-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -16,6 +16,8 @@
 #include <linux/hardirq.h> /* for in_interrupt() */
 #include <linux/hugetlb_inline.h>
 
+struct pagevec;
+
 /*
  * Bits in mapping->flags.
  */
@@ -624,6 +626,8 @@ int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
 extern void delete_from_page_cache(struct page *page);
 extern void __delete_from_page_cache(struct page *page, void *shadow);
 int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask);
+void delete_from_page_cache_batch(struct address_space *mapping,
+				  struct pagevec *pvec);
 
 /*
  * Like add_to_page_cache_locked, but used to add newly allocated pages:

mm/filemap.c

@@ -304,6 +304,89 @@ void delete_from_page_cache(struct page *page)
 }
 EXPORT_SYMBOL(delete_from_page_cache);
 
+/*
+ * page_cache_tree_delete_batch - delete several pages from page cache
+ * @mapping: the mapping to which pages belong
+ * @pvec: pagevec with pages to delete
+ *
+ * The function walks over mapping->page_tree and removes pages passed in @pvec
+ * from the radix tree. The function expects @pvec to be sorted by page index.
+ * It tolerates holes in @pvec (radix tree entries at those indices are not
+ * modified). The function expects only THP head pages to be present in the
+ * @pvec and takes care to delete all corresponding tail pages from the radix
+ * tree as well.
+ *
+ * The function expects mapping->tree_lock to be held.
+ */
+static void
+page_cache_tree_delete_batch(struct address_space *mapping,
+			     struct pagevec *pvec)
+{
+	struct radix_tree_iter iter;
+	void **slot;
+	int total_pages = 0;
+	int i = 0, tail_pages = 0;
+	struct page *page;
+	pgoff_t start;
+
+	start = pvec->pages[0]->index;
+	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
+		if (i >= pagevec_count(pvec) && !tail_pages)
+			break;
+		page = radix_tree_deref_slot_protected(slot,
+						       &mapping->tree_lock);
+		if (radix_tree_exceptional_entry(page))
+			continue;
+		if (!tail_pages) {
+			/*
+			 * Some page got inserted in our range? Skip it. We
+			 * have our pages locked so they are protected from
+			 * being removed.
+			 */
+			if (page != pvec->pages[i])
+				continue;
+			WARN_ON_ONCE(!PageLocked(page));
+			if (PageTransHuge(page) && !PageHuge(page))
+				tail_pages = HPAGE_PMD_NR - 1;
+			page->mapping = NULL;
+			/*
+			 * Leave page->index set: truncation lookup relies
+			 * upon it
+			 */
+			i++;
+		} else {
+			tail_pages--;
+		}
+		radix_tree_clear_tags(&mapping->page_tree, iter.node, slot);
+		__radix_tree_replace(&mapping->page_tree, iter.node, slot, NULL,
+				     workingset_update_node, mapping);
+		total_pages++;
+	}
+	mapping->nrpages -= total_pages;
+}
+
+void delete_from_page_cache_batch(struct address_space *mapping,
+				  struct pagevec *pvec)
+{
+	int i;
+	unsigned long flags;
+
+	if (!pagevec_count(pvec))
+		return;
+
+	spin_lock_irqsave(&mapping->tree_lock, flags);
+	for (i = 0; i < pagevec_count(pvec); i++) {
+		trace_mm_filemap_delete_from_page_cache(pvec->pages[i]);
+
+		unaccount_page_cache_page(mapping, pvec->pages[i]);
+	}
+	page_cache_tree_delete_batch(mapping, pvec);
+	spin_unlock_irqrestore(&mapping->tree_lock, flags);
+
+	for (i = 0; i < pagevec_count(pvec); i++)
+		page_cache_free_page(mapping, pvec->pages[i]);
+}
+
 int filemap_check_errors(struct address_space *mapping)
 {
 	int ret = 0;

mm/truncate.c

@@ -294,6 +294,14 @@ void truncate_inode_pages_range(struct address_space *mapping,
 	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
 			min(end - index, (pgoff_t)PAGEVEC_SIZE),
 			indices)) {
+		/*
+		 * Pagevec array has exceptional entries and we may also fail
+		 * to lock some pages. So we store pages that can be deleted
+		 * in a new pagevec.
+		 */
+		struct pagevec locked_pvec;
+
+		pagevec_init(&locked_pvec, 0);
 		for (i = 0; i < pagevec_count(&pvec); i++) {
 			struct page *page = pvec.pages[i];
 
@@ -315,9 +323,17 @@ void truncate_inode_pages_range(struct address_space *mapping,
 				unlock_page(page);
 				continue;
 			}
-			truncate_inode_page(mapping, page);
-			unlock_page(page);
+			if (page->mapping != mapping) {
+				unlock_page(page);
+				continue;
+			}
+			pagevec_add(&locked_pvec, page);
 		}
+		for (i = 0; i < pagevec_count(&locked_pvec); i++)
+			truncate_cleanup_page(mapping, locked_pvec.pages[i]);
+		delete_from_page_cache_batch(mapping, &locked_pvec);
+		for (i = 0; i < pagevec_count(&locked_pvec); i++)
+			unlock_page(locked_pvec.pages[i]);
 		pagevec_remove_exceptionals(&pvec);
 		pagevec_release(&pvec);
 		cond_resched();