Subject: Re: [PATCH V10 09/19] block: introduce bio_bvecs()
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Dave Chinner <dchinner@redhat.com>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, Shaohua Li <shli@kernel.org>,
        linux-raid@vger.kernel.org, linux-erofs@lists.ozlabs.org,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        linux-xfs@vger.kernel.org, Gao Xiang <gaoxiang25@huawei.com>,
        Theodore Ts'o <tytso@mit.edu>, linux-ext4@vger.kernel.org,
        Coly Li <colyli@suse.de>, linux-bcache@vger.kernel.org,
        Boaz Harrosh <ooo@electrozaur.com>,
        Bob Peterson <rpeterso@redhat.com>, cluster-devel@redhat.com
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 <002fe56b-25e4-573e-c09b-bb12c3e8d25a@grimberg.me>
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 <20181121005902.GA31748@ming.t460p>
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From: Sagi Grimberg <sagi@grimberg.me>
Message-ID: <fc268b0e-61b5-aacc-67be-cb7b266c6d8f@grimberg.me>
Date: Tue, 20 Nov 2018 21:35:07 -0800
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>> 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.