Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758990AbcDFBKg (ORCPT ); Tue, 5 Apr 2016 21:10:36 -0400 Received: from mail-pf0-f194.google.com ([209.85.192.194]:36098 "EHLO mail-pf0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751907AbcDFBKe (ORCPT ); Tue, 5 Apr 2016 21:10:34 -0400 Date: Tue, 5 Apr 2016 17:10:28 -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: <20160406011028.GA32334@kmo-pixel> References: <1459878246-9249-1-git-send-email-ming.lei@canonical.com> <20160406003025.GC31161@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: 1641 Lines: 44 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); } and then call that from the appropriate drivers. It should be like 20 minutes of work. My issue is that your approach of just enforcing a global limit is a step in the wrong direction - we want to get _away_ from that and move towards drivers specifying _directly_ what their limits are: more straightforward, less opaque. Also, your patch is wrong, as it'll break if there's bvecs that aren't full pages.