From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756185AbcDDROF (ORCPT ); Mon, 4 Apr 2016 13:14:05 -0400 Received: from gum.cmpxchg.org ([85.214.110.215]:49038 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754853AbcDDROD (ORCPT ); Mon, 4 Apr 2016 13:14:03 -0400 From: Johannes Weiner To: Andres Freund , Rik van Riel , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: [PATCH 2/3] mm: filemap: only do access activations on reads Date: Mon, 4 Apr 2016 13:13:37 -0400 Message-Id: <1459790018-6630-3-git-send-email-hannes@cmpxchg.org> X-Mailer: git-send-email 2.8.0 In-Reply-To: <1459790018-6630-1-git-send-email-hannes@cmpxchg.org> References: <1459790018-6630-1-git-send-email-hannes@cmpxchg.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andres Freund observed that his database workload is struggling with the transaction journal creating pressure on frequently read pages. Access patterns like transaction journals frequently write the same pages over and over, but in the majority of cases those pages are never read back. There are no caching benefits to be had for those pages, so activating them and having them put pressure on pages that do benefit from caching is a bad choice. Leave page activations to read accesses and don't promote pages based on writes alone. It could be said that partially written pages do contain cache-worthy data, because even if *userspace* does not access the unwritten part, the kernel still has to read it from the filesystem for correctness. However, a counter argument is that these pages enjoy at least *some* protection over other inactive file pages through the writeback cache, in the sense that dirty pages are written back with a delay and cache reclaim leaves them alone until they have been written back to disk. Should that turn out to be insufficient and we see increased read IO from partial writes under memory pressure, we can always go back and update grab_cache_page_write_begin() to take (pos, len) so that it can tell partial writes from pages that don't need partial reads. But for now, keep it simple. Reported-by: Andres Freund Signed-off-by: Johannes Weiner --- mm/filemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/filemap.c b/mm/filemap.c index ca33816..edfec5e 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2579,7 +2579,7 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping, pgoff_t index, unsigned flags) { struct page *page; - int fgp_flags = FGP_LOCK|FGP_ACCESSED|FGP_WRITE|FGP_CREAT; + int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT; if (flags & AOP_FLAG_NOFS) fgp_flags |= FGP_NOFS; -- 2.8.0