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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 8F14EC432BE for ; Mon, 16 Aug 2021 06:28:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 70B7A61A57 for ; Mon, 16 Aug 2021 06:28:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232148AbhHPG2b (ORCPT ); Mon, 16 Aug 2021 02:28:31 -0400 Received: from out0.migadu.com ([94.23.1.103]:25861 "EHLO out0.migadu.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229774AbhHPG2b (ORCPT ); Mon, 16 Aug 2021 02:28:31 -0400 Subject: Re: [PATCH] raid1: ensure bio doesn't have more than BIO_MAX_VECS sectors DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1629095278; 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=axkEiHAnVD5Wd2kbH1btpwM/jQcyRI29qQamEGMsVqA=; b=UhVlWhH8OB9+HrsEMFizsUTdzU15w20HPCbXouscBYmUezSoGtfjf+gvOXs6u9F7BoSP3C qQVn7CC2qHpRZuFySNbTgmwlOMTYaHueOhrM0HHvEQ7q4RyCFI36QxVrQHBH0D1HSwQvrJ mJXDm3tKvGGc/qiZ8NdlzrGb+5Rjq4U= To: Ming Lei , Christoph Hellwig Cc: song@kernel.org, linux-raid@vger.kernel.org, jens@chianterastutte.eu, linux-block@vger.kernel.org References: <20210813060510.3545109-1-guoqing.jiang@linux.dev> <0eac4589-ffd2-fb1a-43cc-87722731438a@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Guoqing Jiang Message-ID: <05bdd906-2e78-bc85-c186-7bffac9076e0@linux.dev> Date: Mon, 16 Aug 2021 14:27:48 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: guoqing.jiang@linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Ming and Christoph, On 8/14/21 4:57 PM, Ming Lei wrote: > On Sat, Aug 14, 2021 at 08:55:21AM +0100, Christoph Hellwig wrote: >> On Fri, Aug 13, 2021 at 04:38:59PM +0800, Guoqing Jiang wrote: >>> Ok, thanks. >>> >>>> In general the size of a bio only depends on the number of vectors, not >>>> the total I/O size. But alloc_behind_master_bio allocates new backing >>>> pages using order 0 allocations, so in this exceptional case the total >>>> size oes actually matter. >>>> >>>> While we're at it: this huge memory allocation looks really deadlock >>>> prone. >>> Hmm, let me think more about it, or could you share your thought? ???? >> Well, you'd need a mempool which can fit the max payload of a bio, >> that is BIO_MAX_VECS pages. IIUC, the behind bio is allocated from bio_set (mddev->bio_set) which is allocated in md_run by call bioset_init, so the mempool (bvec_pool) of  this bio_set is created by biovec_init_pool which uses global biovec slabs. Do we really need another mempool? Or, there is no potential deadlock  for this case. >> FYI, this is what I'd do instead of this patch for now. We don't really >> need a vetor per sector, just per page. So this limits the I/O >> size a little less. >> >> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c >> index 3c44c4bb40fc..5b27d995302e 100644 >> --- a/drivers/md/raid1.c >> +++ b/drivers/md/raid1.c >> @@ -1454,6 +1454,15 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio, >> goto retry_write; >> } >> >> + /* >> + * When using a bitmap, we may call alloc_behind_master_bio below. >> + * alloc_behind_master_bio allocates a copy of the data payload a page >> + * at a time and thus needs a new bio that can fit the whole payload >> + * this bio in page sized chunks. >> + */ Thanks for the above, will copy it accordingly. I will check if WriteMostly is set before, then check both the flag and bitmap. >> + if (bitmap) >> + max_sectors = min_t(int, max_sectors, BIO_MAX_VECS * PAGE_SIZE); > s/PAGE_SIZE/PAGE_SECTORS Agree. >> + >> if (max_sectors < bio_sectors(bio)) { >> struct bio *split = bio_split(bio, max_sectors, >> GFP_NOIO, &conf->bio_split); > Here the limit is max single-page vectors, and the above way may not work, > such as:ust splitted and not > > 0 ~ 254: each bvec's length is 512 > 255: bvec's length is 8192 > > the total length is just 512*255 + 8192 = 138752 bytes = 271 sectors, but it > still may need 257 bvecs, which can't be allocated via bio_alloc_bioset(). Thanks for deeper looking! I guess it is because how vcnt is calculated. > One solution is to add queue limit of max_single_page_bvec, and let > blk_queue_split() handle it. The path (blk_queue_split -> blk_bio_segment_split -> bvec_split_segs) which respects max_segments of limit. Do you mean introduce max_single_page_bvec to limit? Then perform similar checking as for  max_segment. Thanks, Guoqing