linux-kernel - Re: [PATCH 00/17] md: cleanup on direct access to bvec table

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Date:   Thu, 16 Feb 2017 20:16:35 -0800
From:   Shaohua Li <shli@...nel.org>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Jens Axboe <axboe@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
        <linux-raid@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        NeilBrown <neilb@...e.com>
Subject: Re: [PATCH 00/17] md: cleanup on direct access to bvec table

On Fri, Feb 17, 2017 at 09:25:27AM +0800, Ming Lei wrote:
> Hi Shaohua,
> 
> On Fri, Feb 17, 2017 at 6:16 AM, Shaohua Li <shli@...nel.org> wrote:
> > On Thu, Feb 16, 2017 at 07:45:30PM +0800, Ming Lei wrote:
> >> In MD's resync I/O path, there are lots of direct access to bio's
> >> bvec table. This patchset kills most of them, and the conversion
> >> is quite straightforward.
> >
> > I don't like this approach. The MD uses a hacky way to manage pages allocated,
> > this is the root of the problem. The patches add another hack way to do the
> 
> Yes, I agree, and bio_iov_iter_get_pages() uses this kind of hacky way too
> actually.
> 
> > management. I'd like to see explict management of the pages, for example, add
> > data structure in r1bio to manage the pages, then we can use existing API for
> > all the stuffes we need.
> 
> Yeah, that is definitely clean, but we have to pay the following cost:
> 
> - allocate at least N * (128 + 4) bytes per each r1_bio/r10_bio
> - N is pool_info.raid_disks for raid1, and conf->copies for raid10
> 
> If we are happy to introduce the cost, I can take this way in V1.

It's not a big deal. The inflight bio shouldn't be big, so the r1_bio count
isn't big. We don't waste much.

Thanks,
Shaohua