From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751850AbaLYGJ3 (ORCPT ); Thu, 25 Dec 2014 01:09:29 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:38306 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750788AbaLYGJ2 (ORCPT ); Thu, 25 Dec 2014 01:09:28 -0500 MIME-Version: 1.0 In-Reply-To: References: Date: Thu, 25 Dec 2014 14:09:25 +0800 Message-ID: Subject: Re: [RFC PATCH 01/17] block: make generic_make_request handle arbitrarily sized bios From: Ming Lei To: Dongsu Park Cc: Linux Kernel Mailing List , Jens Axboe , Kent Overstreet , Ming Lin , Christoph Hellwig , Al Viro , Neil Brown , Alasdair Kergon , Mike Snitzer , dm-devel@redhat.com, drbd-dev@lists.linbit.com, Jiri Kosina , Geoff Levand , Jim Paris , Joshua Morris , Philip Kelleher , Minchan Kim , Nitin Gupta , Oleg Drokin , Andreas Dilger Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 22, 2014 at 7:48 PM, Dongsu Park wrote: > From: Kent Overstreet > > The way the block layer is currently written, it goes to great lengths > to avoid having to split bios; upper layer code (such as bio_add_page()) > checks what the underlying device can handle and tries to always create > bios that don't need to be split. > > But this approach becomes unwieldy and eventually breaks down with > stacked devices and devices with dynamic limits, and it adds a lot of > complexity. If the block layer could split bios as needed, we could > eliminate a lot of complexity elsewhere - particularly in stacked > drivers. Code that creates bios can then create whatever size bios are > convenient, and more importantly stacked drivers don't have to deal with > both their own bio size limitations and the limitations of the > (potentially multiple) devices underneath them. In the future this will > let us delete merge_bvec_fn and a bunch of other code. Looks it is a very good idea to split bio in block. > > We do this by adding calls to blk_queue_split() to the various > make_request functions that need it - a few can already handle arbitrary I am wondering why the bio isn't splitted just before q->make_request_fn is called in generic_make_request()? By this way, drivers won't need to call blk_queue_split() at all. Is it because performance reason? or others? > size bios. Note that we add the call _after_ any call to > blk_queue_bounce(); this means that blk_queue_split() and > blk_recalc_rq_segments() don't need to be concerned with bouncing > affecting segment merging. > > Some make_request_fn() callbacks were simple enough to audit and verify > they don't need blk_queue_split() calls. The skipped ones are: > > * nfhd_make_request (arch/m68k/emu/nfblock.c) > * axon_ram_make_request (arch/powerpc/sysdev/axonram.c) > * simdisk_make_request (arch/xtensa/platforms/iss/simdisk.c) > * brd_make_request (ramdisk - drivers/block/brd.c) > * mtip_submit_request (drivers/block/mtip32xx/mtip32xx.c) > * loop_make_request > * null_queue_bio > * bcache's make_request fns I guess the above drivers haven't max_sectors/max_segment limit. Thanks, Ming Lei