From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A8793C433E0 for ; Mon, 4 Jan 2021 16:04:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 75DFE2072D for ; Mon, 4 Jan 2021 16:04:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727900AbhADQD4 (ORCPT ); Mon, 4 Jan 2021 11:03:56 -0500 Received: from mail.kernel.org ([198.145.29.99]:41406 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728024AbhADQDw (ORCPT ); Mon, 4 Jan 2021 11:03:52 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id B7B7E207AE; Mon, 4 Jan 2021 16:03:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1609776191; bh=JQtq7FxcglCU79ILboju5l5AL9giFdw3TA6cxNOZBlU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WTQxaeVZneErTrlFNv0VpJltv6B6rfvC1B5xymxXcoaM/OBKQMeGnr5FVJMXNjkhf 7oOEJyNNH0iU72p5G4LiNDVFW5i+ct+hbn/AAkVd3z8bbZ8pmRs1RVcApyzPp+/iol cyxtu925y/crOmewliE+aYSF2jX7He8DF5P9RNmw= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Tosk Robot , Chunguang Xu , Samuel Liao , Andreas Dilger , Theodore Tso , Sasha Levin Subject: [PATCH 5.10 62/63] ext4: avoid s_mb_prefetch to be zero in individual scenarios Date: Mon, 4 Jan 2021 16:57:55 +0100 Message-Id: <20210104155711.810538946@linuxfoundation.org> X-Mailer: git-send-email 2.30.0 In-Reply-To: <20210104155708.800470590@linuxfoundation.org> References: <20210104155708.800470590@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Chunguang Xu [ Upstream commit 82ef1370b0c1757ab4ce29f34c52b4e93839b0aa ] Commit cfd732377221 ("ext4: add prefetching for block allocation bitmaps") introduced block bitmap prefetch, and expects to read block bitmaps of flex_bg through an IO. However, it seems to ignore the value range of s_log_groups_per_flex. In the scenario where the value of s_log_groups_per_flex is greater than 27, s_mb_prefetch or s_mb_prefetch_limit will overflow, cause a divide zero exception. In addition, the logic of calculating nr is also flawed, because the size of flexbg is fixed during a single mount, but s_mb_prefetch can be modified, which causes nr to fail to meet the value condition of [1, flexbg_size]. To solve this problem, we need to set the upper limit of s_mb_prefetch. Since we expect to load block bitmaps of a flex_bg through an IO, we can consider determining a reasonable upper limit among the IO limit parameters. After consideration, we chose BLK_MAX_SEGMENT_SIZE. This is a good choice to solve divide zero problem and avoiding performance degradation. [ Some minor code simplifications to make the changes easy to follow -- TYT ] Reported-by: Tosk Robot Signed-off-by: Chunguang Xu Reviewed-by: Samuel Liao Reviewed-by: Andreas Dilger Link: https://lore.kernel.org/r/1607051143-24508-1-git-send-email-brookxu@tencent.com Signed-off-by: Theodore Ts'o Signed-off-by: Sasha Levin --- fs/ext4/mballoc.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 37a619bf1ac7c..e67d5de6f28ca 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -2395,9 +2395,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac) nr = sbi->s_mb_prefetch; if (ext4_has_feature_flex_bg(sb)) { - nr = (group / sbi->s_mb_prefetch) * - sbi->s_mb_prefetch; - nr = nr + sbi->s_mb_prefetch - group; + nr = 1 << sbi->s_log_groups_per_flex; + nr -= group & (nr - 1); + nr = min(nr, sbi->s_mb_prefetch); } prefetch_grp = ext4_mb_prefetch(sb, group, nr, &prefetch_ios); @@ -2733,7 +2733,8 @@ static int ext4_mb_init_backend(struct super_block *sb) if (ext4_has_feature_flex_bg(sb)) { /* a single flex group is supposed to be read by a single IO */ - sbi->s_mb_prefetch = 1 << sbi->s_es->s_log_groups_per_flex; + sbi->s_mb_prefetch = min(1 << sbi->s_es->s_log_groups_per_flex, + BLK_MAX_SEGMENT_SIZE >> (sb->s_blocksize_bits - 9)); sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */ } else { sbi->s_mb_prefetch = 32; -- 2.27.0