Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Subject: [PATCH] mm: move more expensive part of XA setup out of mapping check
Message-ID: <49f67983-b802-8929-edab-d807f745c9ca@kernel.dk>
Date:   Tue, 26 Oct 2021 10:01:20 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

The fast path here is not needing any writeback, yet we spend time setting
up the xarray lookup data upfront. Move the part that actually needs to
iterate the address space mapping into a separate helper, saving ~30% of
the time here.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/mm/filemap.c b/mm/filemap.c
index 850920276846..0e4021edac0b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -638,6 +638,30 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 	return mapping->nrpages;
 }
 
+static bool filemap_range_has_writeback(struct address_space *mapping,
+					loff_t start_byte, loff_t end_byte)
+{
+	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
+	pgoff_t max = end_byte >> PAGE_SHIFT;
+	struct page *page;
+
+	if (end_byte < start_byte)
+		return false;
+
+	rcu_read_lock();
+	xas_for_each(&xas, page, max) {
+		if (xas_retry(&xas, page))
+			continue;
+		if (xa_is_value(page))
+			continue;
+		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
+			break;
+	}
+	rcu_read_unlock();
+	return page != NULL;
+
+}
+
 /**
  * filemap_range_needs_writeback - check if range potentially needs writeback
  * @mapping:           address space within which to check
@@ -655,29 +679,12 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 bool filemap_range_needs_writeback(struct address_space *mapping,
 				   loff_t start_byte, loff_t end_byte)
 {
-	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
-	pgoff_t max = end_byte >> PAGE_SHIFT;
-	struct page *page;
-
 	if (!mapping_needs_writeback(mapping))
 		return false;
 	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
 	    !mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
 		return false;
-	if (end_byte < start_byte)
-		return false;
-
-	rcu_read_lock();
-	xas_for_each(&xas, page, max) {
-		if (xas_retry(&xas, page))
-			continue;
-		if (xa_is_value(page))
-			continue;
-		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
-			break;
-	}
-	rcu_read_unlock();
-	return page != NULL;
+	return filemap_range_has_writeback(mapping, start_byte, end_byte);
 }
 EXPORT_SYMBOL_GPL(filemap_range_needs_writeback);
 
-- 
Jens Axboe