From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755569Ab0CCSpM (ORCPT ); Wed, 3 Mar 2010 13:45:12 -0500 Received: from mail-bw0-f209.google.com ([209.85.218.209]:34275 "EHLO mail-bw0-f209.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755416Ab0CCSpK convert rfc822-to-8bit (ORCPT ); Wed, 3 Mar 2010 13:45:10 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=sender:from:to:cc:subject:references:date:in-reply-to:message-id :user-agent:mime-version:content-type:content-transfer-encoding; b=Ha4RA2p4H1rH+U3TpGnAWYdPVtEed7S/b1PwMaw9j4YG5YKzNCagzBXkAkZOsaKmV3 eTGrw5t6RnpmkiE0GBM3q4ufMu21tRcSNyaj6dZpCRGcBFqgByRwCQJ8e8tJQDFBrpSZ N04iNDnEkZhGsOnoT6SbNNUcFq9aThoJd6/r4= From: Dmitry Monakhov To: Mike Snitzer Cc: Jens Axboe , linux-kernel@vger.kernel.org, dm-devel@redhat.com Subject: Re: [PATCH 1/2] blkdev: fix merge_bvec_fn return value checks References: <1267292113-12900-1-git-send-email-dmonakhov@openvz.org> <20100228184634.GI5768@kernel.dk> <874okyf4iw.fsf@openvz.org> <170fa0d21003031020x5b71b492vd733cf0d7c9b83d4@mail.gmail.com> Date: Wed, 03 Mar 2010 21:45:05 +0300 In-Reply-To: <170fa0d21003031020x5b71b492vd733cf0d7c9b83d4@mail.gmail.com> (Mike Snitzer's message of "Wed, 3 Mar 2010 13:20:23 -0500") Message-ID: <87wrxtkzwu.fsf@openvz.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.50 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mike Snitzer writes: > On Tue, Mar 2, 2010 at 10:49 PM, Dmitry Monakhov wrote: >> Jens Axboe writes: >> >>> On Sat, Feb 27 2010, Dmitry Monakhov wrote: >>>> merge_bvec_fn() returns bvec->bv_len on success. So we have to check >>>> against this value. But in case of fs_optimization merge we compare >>>> with wrong value. This patch must be included in >>>>  b428cd6da7e6559aca69aa2e3a526037d3f20403 >>>> But accidentally i've forgot to add this in the initial patch. >>>> To make things straight let's replace all such checks. >>>> In fact this makes code easy to understand. >>> >>> Agree, applied. >> Ohh.. as you already know this patch break dm-layer. Sorry. >> This is because dm->merge may return more than requested. So correct >> check must test against less what requested. Correct patch attached. > > Yes, it is quite common for dm_merge_bvec() to return greater than the > requested length. > > But dm_merge_bvec() returning a maximum length, rather than requested, > isn't special. All the other blk_queue_merge_bvec() callers' merge > functions appear to return "maximum amount of bytes we can accept at > this offset" too. What for? Does it allow us to make some optimization? For example like follows: bio_add_pageS(bio, **pages) { /* call merge_fn only one untill all space exhausted */ ret = merge_fn() (this returns huge value (1024*1024)) while (ret) { bio->bi_io_vec[bio->bi_vcnt - 1].bv_page = page; ... ret -= PAGE_SIZE; bio->bi_vcnt++; } } IMHO the answer is *NO*, this code will unlikely to work. > > __bio_add_page() only needs to care about whether the > 'q->merge_bvec_fn' return is _less than_ the requested length. > >> From 145fb49bf2251f445ca29c5218333367448932d6 Mon Sep 17 00:00:00 2001 >> From: Dmitry Monakhov >> Date: Wed, 3 Mar 2010 06:28:06 +0300 >> Subject: [PATCH] blkdev: fix merge_bvec_fn return value checks v2 >> >> merge_bvec_fn() returns bvec->bv_len on success. So we have to check >> against this value. But in case of fs_optimization merge we compare >> with wrong value. This patch must be included in >>  b428cd6da7e6559aca69aa2e3a526037d3f20403 >> But accidentally i've forgot to add this in the initial patch. >> To make things straight let's replace all such checks. >> In fact this makes code easy to understand. >> >> Signed-off-by: Dmitry Monakhov >> --- >>  fs/bio.c |    4 ++-- >>  1 files changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/fs/bio.c b/fs/bio.c >> index 88094af..975657a 100644 >> --- a/fs/bio.c >> +++ b/fs/bio.c >> @@ -557,7 +557,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page >>                                        .bi_rw = bio->bi_rw, >>                                }; >> >> -                               if (q->merge_bvec_fn(q, &bvm, prev) < len) { >> +                               if (q->merge_bvec_fn(q, &bvm, prev) < prev->bv_len) { >>                                        prev->bv_len -= len; >>                                        return 0; >>                                } >> @@ -611,7 +611,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page >>                 * merge_bvec_fn() returns number of bytes it can accept >>                 * at this offset >>                 */ >> -               if (q->merge_bvec_fn(q, &bvm, bvec) < len) { >> +               if (q->merge_bvec_fn(q, &bvm, bvec) < bvec->bv_len) { >>                        bvec->bv_page = NULL; >>                        bvec->bv_len = 0; >>                        bvec->bv_offset = 0; > > NOTE this 2nd hunk doesn't change anything at all because: bvec->bv_len = len; Yess. IMHO this makes code more readable. > > Mike