linux-ext4 - Re: [PATCH V10 09/19] block: introduce bio

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Date:   Tue, 20 Nov 2018 21:35:07 -0800
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Ming Lei <ming.lei@...hat.com>
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()


>> 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.