linux-kernel - Re: [PATCH V10 09/19] block: introduce bio

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Date:   Wed, 21 Nov 2018 18:19:38 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Coly Li <colyli@...e.de>, linux-bcache@...r.kernel.org,
        Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V10 09/19] block: introduce bio_bvecs()

On Tue, Nov 20, 2018 at 09:35:07PM -0800, Sagi Grimberg wrote:
> 
> > > Wait, I see that the bvec is still a single array per bio. When you said
> > > a table I thought you meant a 2-dimentional array...
> > 
> > I mean a new 1-d table A has to be created for multiple bios in one rq,
> > and build it in the following way
> > 
> >             rq_for_each_bvec(tmp, rq, rq_iter)
> >                      *A = tmp;
> > 
> > Then you can pass A to iov_iter_bvec() & send().
> > 
> > Given it is over TCP, I guess it should be doable for you to preallocate one
> > 256-bvec table in one page for each request, then sets the max segment size as
> > (unsigned int)-1, and max segment number as 256, the preallocated table
> > should work anytime.
> 
> 256 bvec table is really a lot to preallocate, especially when its not
> needed, I can easily initialize the bvec_iter on the bio bvec. If this
> involves preallocation of the worst-case than I don't consider this to
> be an improvement.

If you don't provide one single bvec table, I understand you may not send
this req via one send().

The bvec_iter initialization is easy to do:

	bvec_iter = bio->bi_iter

when you move to a new a bio, please refer to  __bio_for_each_bvec() or
__bio_for_each_segment().

Thanks,
Ming