Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759886AbcDFCW4 (ORCPT ); Tue, 5 Apr 2016 22:22:56 -0400 Received: from mail-pa0-f66.google.com ([209.85.220.66]:35160 "EHLO mail-pa0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754605AbcDFCWy (ORCPT ); Tue, 5 Apr 2016 22:22:54 -0400 Date: Tue, 5 Apr 2016 18:22:49 -0800 From: Kent Overstreet To: Ming Lei Cc: Jens Axboe , Linux Kernel Mailing List , linux-block@vger.kernel.org, Christoph Hellwig , Eric Wheeler , Sebastian Roesner , "4.2+" , Shaohua Li Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs Message-ID: <20160406022249.GB7452@kmo-pixel> References: <1459878246-9249-1-git-send-email-ming.lei@canonical.com> <20160406003025.GC31161@kmo-pixel> <20160406011028.GA32334@kmo-pixel> <20160406012839.GA32713@kmo-pixel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2503 Lines: 58 On Wed, Apr 06, 2016 at 09:51:02AM +0800, Ming Lei wrote: > On Wed, Apr 6, 2016 at 9:28 AM, Kent Overstreet > wrote: > > On Wed, Apr 06, 2016 at 09:20:59AM +0800, Ming Lei wrote: > >> On Wed, Apr 6, 2016 at 9:10 AM, Kent Overstreet > >> wrote: > >> > On Wed, Apr 06, 2016 at 08:59:31AM +0800, Ming Lei wrote: > >> >> On Wed, Apr 6, 2016 at 8:30 AM, Kent Overstreet > >> >> wrote: > >> >> > On Wed, Apr 06, 2016 at 01:44:06AM +0800, Ming Lei wrote: > >> >> >> After arbitrary bio size is supported, the incoming bio may > >> >> >> be very big. We have to split the bio into small bios so that > >> >> >> each holds at most BIO_MAX_PAGES bvecs for safety reason, such > >> >> >> as bio_clone(). > >> >> >> > >> >> >> This patch fixes the following kernel crash: > >> >> > > >> >> > Ming, let's not do it this way; drivers that don't clone biovecs are the norm - > >> >> > instead, md has its own queue limits that it ought to be setting up correctly. > >> >> > >> >> Except for md, there are also several usages of bio_clone: > >> >> > >> >> - drbd > >> >> - osdblk > >> >> - pktcdvd > >> >> - xen-blkfront > >> >> - verify code of bcache > >> >> > >> >> I don't like bio_clone() too, which can cause trouble to multipage bvecs. > >> >> > >> >> How about fixing the issue by this simple patch first? Then once we limits > >> >> all above queues by max sectors, the global limit can be removed as > >> >> mentioned by the comment. > >> > > >> > just do this: > >> > > >> > void blk_set_limit_clonable(struct queue_limits *lim) > >> > { > >> > lim->max_segments = min(lim->max_segments, BIO_MAX_PAGES); > >> > } > >> > >> As I memtioned it is __not__ correct to use max_segments, and the issue is > >> related with max sectors, please see the code of bio_clone_bioset(): > > > > I know how bio_clone_bioset() works but I'm not seeing how that has anything to > > do with max sectors. The way it copies the biovec is not going to merge > > segments, if the original bio had non full page segments then so is the clone. > > OK, I see, now it is a totally new limit, and no current queue limit can fit > the purpose. > > Looks we need to introduce the new limit of io_max_vecs, which can be > applied into blk_bio_segment_split(). > > But a queue flag should be better than queue limit since it is a 'limit' from > software/driver. Why is max_segments not appropriate?