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 ABB88C433EF for ; Thu, 26 May 2022 01:16:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234279AbiEZBQX (ORCPT ); Wed, 25 May 2022 21:16:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51678 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229917AbiEZBQV (ORCPT ); Wed, 25 May 2022 21:16:21 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E4F5A4EA36; Wed, 25 May 2022 18:16:19 -0700 (PDT) Received: from dggpeml500020.china.huawei.com (unknown [172.30.72.56]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4L7qgc2f40zQkNm; Thu, 26 May 2022 09:13:16 +0800 (CST) Received: from [10.174.177.174] (10.174.177.174) by dggpeml500020.china.huawei.com (7.185.36.88) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Thu, 26 May 2022 09:16:16 +0800 Message-ID: <850b644f-a5b5-48d8-8899-7de7bd944745@huawei.com> Date: Thu, 26 May 2022 09:16:16 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.8.1 Subject: Re: [PATCH 2/2] ext4: correct the judgment of BUG in ext4_mb_normalize_request To: Jan Kara CC: , , , , , , , , , Baokun Li References: <20220521134217.312071-1-libaokun1@huawei.com> <20220521134217.312071-3-libaokun1@huawei.com> <20220523094023.e3rnile4wh7uiich@quack3.lan> <3755e40b-f817-83df-b239-b0697976c272@huawei.com> <20220524093026.qhwyibhgg6ulsw6r@quack3.lan> <26962b95-1129-60c4-dbde-6fea44c514a6@huawei.com> <20220525112932.d3gi7nynygkirdpi@quack3.lan> From: Baokun Li In-Reply-To: <20220525112932.d3gi7nynygkirdpi@quack3.lan> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.174] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpeml500020.china.huawei.com (7.185.36.88) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org 在 2022/5/25 19:29, Jan Kara 写道: > On Tue 24-05-22 21:44:31, Baokun Li wrote: >> 在 2022/5/24 17:30, Jan Kara 写道: >>> On Mon 23-05-22 21:04:16, libaokun (A) wrote: >>>> 在 2022/5/23 17:40, Jan Kara 写道: >>>>> On Sat 21-05-22 21:42:17, Baokun Li wrote: >>>>>> When either of the "start + size <= ac->ac_o_ex.fe_logical" or >>>>>> "start > ac->ac_o_ex.fe_logical" conditions is met, it indicates >>>>>> that the fe_logical is not in the allocated range. >>>>>> In this case, it should be bug_ON. >>>>>> >>>>>> Fixes: dfe076c106f6 ("ext4: get rid of code duplication") >>>>>> Signed-off-by: Baokun Li >>>>> I think this is actually wrong. The original condition checks whether >>>>> start + size does not overflow the used integer type. Your condition is >>>>> much stronger and I don't think it always has to be true. E.g. allocation >>>>> goal block (start variable) can be pushed to larger values by existing >>>>> preallocation or so. >>>>> >>>>> Honza >>>>> >>>> I think there are two reasons for this: >>>> >>>> First of all, the code here is as follows. >>>> ``` >>>>         size = end - start; >>>>         [...] >>>> if (start + size <= ac->ac_o_ex.fe_logical && >>>>                         start > ac->ac_o_ex.fe_logical) { >>>>                 ext4_msg(ac->ac_sb, KERN_ERR, >>>>                          "start %lu, size %lu, fe_logical %lu", >>>>                          (unsigned long) start, (unsigned long) size, >>>>                          (unsigned long) ac->ac_o_ex.fe_logical); >>>> BUG(); >>>> } >>>>         BUG_ON(size <= 0 || size > EXT4_BLOCKS_PER_GROUP(ac->ac_sb)); >>>> ``` >>>> First of all, there is no need to compare with ac_o_ex.fe_logical if it is >>>> to determine whether there is an overflow. >>>> Because the previous logic guarantees start < = ac_o_ex.fe_logical, and >>> How does it guarantee that? The logic: >>> >>> if (ar->pleft && start <= ar->lleft) { >>> size -= ar->lleft + 1 - start; >>> start = ar->lleft + 1; >>> } >>> >>> can move 'start' to further blocks... >> This is not the case. According to the code of the preceding process, >> ar->pleft and ar->pright are assigned values in ext4_ext_map_blocks. >> ar->pleft is the first allocated block found to the left by map->m_lblk >> (that is, fe_logical), >> and ar->pright is the first allocated block found to the right. >> ar->lleft and ar->lright are logical block numbers, so there must be >> "ar->lleft < ac_o_ex.fe_logical < ar->lright". > Right, I've found that out after sending my previous email. Sorry for > confusion. Don't be sorry. Thank you very much for your advice. It has benefited me a lot. > >>>> Secondly, the following code flow also reflects this logic. >>>> >>>>            ext4_mb_normalize_request >>>>             >>> start + size <= ac->ac_o_ex.fe_logical >>>>            ext4_mb_regular_allocator >>>>             ext4_mb_simple_scan_group >>>>              ext4_mb_use_best_found >>>>               ext4_mb_new_preallocation >>>>                ext4_mb_new_inode_pa >>>>                 ext4_mb_use_inode_pa >>>>                  >>> set ac->ac_b_ex.fe_len <= 0 >>>>            ext4_mb_mark_diskspace_used >>>>             >>> BUG_ON(ac->ac_b_ex.fe_len <= 0); >>>> >>>> In ext4_mb_use_inode_pa, you have the following code. >>>> ``` >>>> start = pa->pa_pstart + (ac->ac_o_ex.fe_logical - pa->pa_lstart); >>>> end = min(pa->pa_pstart + EXT4_C2B(sbi, pa->pa_len), start + EXT4_C2B(sbi, >>>> ac->ac_o_ex.fe_len)); >>>> len = EXT4_NUM_B2C(sbi, end - start); >>>> ac->ac_b_ex.fe_len = len; >>>> ``` >>>> The starting position in ext4_mb_mark_diskspace_used will be assert. >>>> BUG_ON(ac->ac_b_ex.fe_len <= 0); >>>> When end == start + EXT4_C2B(sbi, ac->ac_o_ex.fe_len) is used, the value of >>>> end - start must be greater than 0. >>>> However, when end == pa->pa_pstart + EXT4_C2B(sbi, pa->pa_len) occurs, this >>>> bug_ON may be triggered. >>>> When this bug_ON is triggered, that is, >>>> >>>> ac->ac_b_ex.fe_len <= 0 >>>> end - start <= 0 >>>> end <= start >>>> pa->pa_pstart + EXT4_C2B(sbi, pa->pa_len) <= pa->pa_pstart + >>>> (ac->ac_o_ex.fe_logical - pa->pa_lstart) >>>> pa->pa_len <= ac->ac_o_ex.fe_logical - pa->pa_lstart >>>> pa->pa_lstart + pa->pa_len <= ac->ac_o_ex.fe_logical >>>> start + size <= ac->ac_o_ex.fe_logical >>>> >>>> So I think that "&&" here should be changed to "||". >>> Sorry, I still disagree. After some more code reading I agree that >>> ac->ac_o_ex.fe_logical is the logical block where we want allocated blocks >>> to be placed in the inode so logical extent of allocated blocks should include >>> ac->ac_o_ex.fe_logical. But I would be reluctant to make assertion you >>> suggest unless we make sure ac->ac_o_ex.fe_logical in unallocated (in which >>> case we can also remove some other code from ext4_mb_normalize_request()). >>> >>> Honza >>> >> What codes are you referring to that can be deleted? > So I though the shifting of 'start' by lleft cannot happen but then I > realized that if 'start' got aligned down, it can now be lower than lleft > so the shifting is indeed needed. So all code is needed there. > > Honza Okay, thanks again! -- With Best Regards, Baokun Li .