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Date:   Wed, 21 Nov 2018 09:46:08 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Sagi Grimberg <sagi@grimberg.me>
Cc:     Ming Lei <ming.lei@redhat.com>, 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,
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        Boaz Harrosh <ooo@electrozaur.com>,
        Bob Peterson <rpeterso@redhat.com>, cluster-devel@redhat.com
Subject: Re: [PATCH V10 09/19] block: introduce bio_bvecs()
Message-ID: <20181121084608.GA29622@lst.de>
References: <002fe56b-25e4-573e-c09b-bb12c3e8d25a@grimberg.me> <20181120161651.GB2629@lst.de> <53526aae-fb9b-ee38-0a01-e5899e2d4e4d@grimberg.me> <20181121005902.GA31748@ming.t460p> <2d9bee7a-f010-dcf4-1184-094101058584@grimberg.me> <20181121034415.GA8408@ming.t460p> <2a47d336-c19b-6bf4-c247-d7382871eeea@grimberg.me> <7378bf49-5a7e-5622-d4d1-808ba37ce656@grimberg.me> <20181121050359.GA31915@ming.t460p> <fc268b0e-61b5-aacc-67be-cb7b266c6d8f@grimberg.me>
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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.