Date: Wed, 21 Nov 2018 13:04:00 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Sagi Grimberg <sagi@grimberg.me>
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
Subject: Re: [PATCH V10 09/19] block: introduce bio_bvecs()
Message-ID: <20181121050359.GA31915@ming.t460p>
References: <20181115085306.9910-10-ming.lei@redhat.com>
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 <20181121005902.GA31748@ming.t460p>
 <2d9bee7a-f010-dcf4-1184-094101058584@grimberg.me>
 <20181121034415.GA8408@ming.t460p>
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On Tue, Nov 20, 2018 at 08:42:04PM -0800, Sagi Grimberg wrote:
> 
> > > Yeah, that is the most common example, given merge is enabled
> > > in most of cases. If the driver or device doesn't care merge,
> > > you can disable it and always get single bio request, then the
> > > bio's bvec table can be reused for send().
> > 
> > Does bvec_iter span bvecs with your patches? I didn't see that change?
> 
> 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.


Thanks,
Ming