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

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Message-ID: <20181121084608.GA29622@lst.de>
Date:   Wed, 21 Nov 2018 09:46:08 +0100
From:   Christoph Hellwig <hch@....de>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Ming Lei <ming.lei@...hat.com>, 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:
>> 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.

Ok, I took a look at the nvme-tcp code and it seems you care about
bios because you want a contiguos bio chunk for sending it down
the networking code.  Yes, in that case we sort of need to iterate
over bios.  But you already have a special case for discard, so you
don't really need any of the magic in the bio_bvecs() helper either
can can just count bi_vcnt in the bio.