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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2F9ABC001B0 for ; Wed, 28 Jun 2023 01:39:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230451AbjF1Bjo (ORCPT ); Tue, 27 Jun 2023 21:39:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49000 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229861AbjF1Bjn (ORCPT ); Tue, 27 Jun 2023 21:39:43 -0400 X-Greylist: delayed 386 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Tue, 27 Jun 2023 18:39:41 PDT Received: from out-26.mta1.migadu.com (out-26.mta1.migadu.com [IPv6:2001:41d0:203:375::1a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B999010FF for ; Tue, 27 Jun 2023 18:39:41 -0700 (PDT) Message-ID: <9db7b307-11c7-2770-786d-5727d771aaa9@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1687915993; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/C2/e9OXN1uWKWIpZoEkIk0l2No59LPS3LVXKbpNiKw=; b=np7oTgs6/QoAHAGzSbaUg+u/r13kffSUylCb3S7Sc4I/DmYNP5QxmisSgAqx3ATWRIxzwO EiGMlqhXcn1T8qCzcm+tjJ/dC/RVkWbOUd5ZakaqNF9U4fKo7dd69z7Vt6LTh7XVhjY0+m gKPDEMWgY/N4L502LCeMvhppiEXvQp4= Date: Wed, 28 Jun 2023 09:33:19 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs To: Ojaswin Mujoo Cc: linux-ext4@vger.kernel.org, Theodore Ts'o , Ritesh Harjani , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Jan Kara , Kemeng Shi , Ritesh Harjani References: <05e648ae04ec5b754207032823e9c1de9a54f87a.1685449706.git.ojaswin@linux.ibm.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Guoqing Jiang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hi Ojaswin, On 6/27/23 14:51, Ojaswin Mujoo wrote: > On Tue, Jun 06, 2023 at 10:00:57PM +0800, Guoqing Jiang wrote: >> Hello, >> >> On 5/30/23 20:33, Ojaswin Mujoo wrote: >>> Before this patch, the call stack in ext4_run_li_request is as follows: >>> >>> /* >>> * nr = no. of BGs we want to fetch (=s_mb_prefetch) >>> * prefetch_ios = no. of BGs not uptodate after >>> * ext4_read_block_bitmap_nowait() >>> */ >>> next_group = ext4_mb_prefetch(sb, group, nr, prefetch_ios); >>> ext4_mb_prefetch_fini(sb, next_group prefetch_ios); >>> >>> ext4_mb_prefetch_fini() will only try to initialize buddies for BGs in >>> range [next_group - prefetch_ios, next_group). This is incorrect since >>> sometimes (prefetch_ios < nr), which causes ext4_mb_prefetch_fini() to >>> incorrectly ignore some of the BGs that might need initialization. This >>> issue is more notable now with the previous patch enabling "fetching" of >>> BLOCK_UNINIT BGs which are marked buffer_uptodate by default. >>> >>> Fix this by passing nr to ext4_mb_prefetch_fini() instead of >>> prefetch_ios so that it considers the right range of groups. >> Thanks for the series. >> >>> Similarly, make sure we don't pass nr=0 to ext4_mb_prefetch_fini() in >>> ext4_mb_regular_allocator() since we might have prefetched BLOCK_UNINIT >>> groups that would need buddy initialization. >> Seems ext4_mb_prefetch_fini can't be called by ext4_mb_regular_allocator >> if nr is 0. > Hi Guoqing, > > Sorry I was on vacation so didn't get a chance to reply to this sooner. > Let me explain what I meant by that statement in the commit message. > > So basically, the prefetch_ios output argument is incremented whenever > ext4_mb_prefetch() reads a block group with !buffer_uptodate(bh). > However, for BLOCK_UNINIT BGs the buffer is marked uptodate after > initialization and hence prefetch_ios is not incremented when such BGs > are prefetched. > > This leads to nr becoming 0 due to the following line (removed in this patch): > > if (prefetch_ios == curr_ios) > nr = 0; > > hence ext4_mb_prefetch_fini() would never pre initialise the corresponding > buddy structures. Instead, these structures would then get initialized > probably at a later point during the slower allocation criterias. The > motivation of making sure the BLOCK_UNINIT BGs' buddies are pre > initialized is so the faster allocation criterias can utilize the data > to make better decisions. Got it, appreciate for the detailed explanation! Thanks, Guoqing