From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zimbra.linbit.com (zimbra.linbit.com [212.69.161.123]) by mail09.linbit.com (LINBIT Mail Daemon) with ESMTP id 51EF1105619A for ; Fri, 10 Aug 2012 22:33:33 +0200 (CEST) Received: from localhost (localhost [127.0.0.1]) by zimbra.linbit.com (Postfix) with ESMTP id 4B1A31B435C for ; Fri, 10 Aug 2012 22:33:33 +0200 (CEST) Received: from zimbra.linbit.com ([127.0.0.1]) by localhost (zimbra.linbit.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id fsuMzM4JO8pi for ; Fri, 10 Aug 2012 22:33:33 +0200 (CEST) Received: from soda.linbit (tuerlsteher.linbit.com [86.59.100.100]) by zimbra.linbit.com (Postfix) with ESMTP id 31AC91B4206 for ; Fri, 10 Aug 2012 22:33:33 +0200 (CEST) Resent-Message-ID: <20120810203333.GF31190@soda.linbit> Received: from mail-yw0-f54.google.com (mail-yw0-f54.google.com [209.85.213.54]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (No client certificate requested) by mail09.linbit.com (LINBIT Mail Daemon) with ESMTPS id 28AFE1045C2E for ; Thu, 9 Aug 2012 05:07:13 +0200 (CEST) Received: by yhfs35 with SMTP id s35so2323847yhf.27 for ; Wed, 08 Aug 2012 20:07:12 -0700 (PDT) Date: Wed, 8 Aug 2012 20:06:38 -0700 From: Kent Overstreet To: Tejun Heo Message-ID: <20120809030638.GL7262@moria.home.lan> References: <1344290921-25154-1-git-send-email-koverstreet@google.com> <1344290921-25154-13-git-send-email-koverstreet@google.com> <20120808233007.GM6983@dhcp-172-17-108-109.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120808233007.GM6983@dhcp-172-17-108-109.mtv.corp.google.com> Cc: axboe@kernel.dk, dm-devel@redhat.com, neilb@suse.de, linux-kernel@vger.kernel.org, linux-bcache@vger.kernel.org, mpatocka@redhat.com, vgoyal@redhat.com, yehuda@hq.newdream.net, sage@newdream.net, agk@redhat.com, drbd-dev@lists.linbit.com Subject: Re: [Drbd-dev] [PATCH v5 12/12] block: Only clone bio vecs that are in use List-Id: Coordination of development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Aug 08, 2012 at 04:30:07PM -0700, Tejun Heo wrote: > Hello, > > On Mon, Aug 06, 2012 at 03:08:41PM -0700, Kent Overstreet wrote: > > @@ -459,10 +460,10 @@ void __bio_clone(struct bio *bio, struct bio *bio_src) > > bio->bi_sector = bio_src->bi_sector; > > bio->bi_bdev = bio_src->bi_bdev; > > bio->bi_flags |= 1 << BIO_CLONED; > > + bio->bi_flags &= ~(1 << BIO_SEG_VALID); > > This isn't obvious at all. Why no explanation anywhere? Also it > would be nice to update comments of the updated functions so that it's > clear that only partial cloning happens. Because it's not obvious to me, either - I had to grep around through a fair amount of code to figure out the semantics of BIO_SEG_VALID and I doubt I have it 100%. I'm also pretty sure it's not used consistently in the existing code... If it means what I think it means, it should be obvious - nr_segs isn't valid because the number of pages in the iovec changed (though we didn't bother to copy it over anyways. So it's not necessary if nr_segs = 0 means nr_segs isn't valid, but bleh). Anyways. yeah. BIO_SEG_VALID should be documented, and if it was I think this code would be fine. I will update the comment for the partial cloning thing: /** * __bio_clone - clone a bio * @bio: destination bio * @bio_src: bio to clone * * Clone a &bio. Caller will own the returned bio, but not * the actual data it points to. Reference count of returned * bio will be one. * * We don't clone the entire bvec, just the part from bi_idx to b_vcnt * (i.e. what the bio currently points to, so the new bio is still * equivalent to the old bio). */ void __bio_clone(struct bio *bio, struct bio *bio_src) { memcpy(bio->bi_io_vec, bio_iovec(bio_src), bio_segments(bio_src) * sizeof(struct bio_vec));