linux-kernel - Re: [PATCH 26/60] btrfs: set NO_MP for request queues behind BTRFS

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Message-ID: <20161031153644.GK30919@infradead.org>
Date:   Mon, 31 Oct 2016 08:36:44 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
        David Sterba <dsterba@...e.com>,
        "open list:BTRFS FILE SYSTEM" <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH 26/60] btrfs: set NO_MP for request queues behind BTRFS

On Sat, Oct 29, 2016 at 04:08:25PM +0800, Ming Lei wrote:
> There are lots of direct access to .bi_vcnt & .bi_io_vec
> of bio, and it isn't ready to support multipage bvecs
> for BTRFS, so set NO_MP for these request queues.

For one bio is an I/O submitter, it has absolutely no business changing
queue flags - if we need to stick to this limitation it simply needs
a version of bio_add_page that doesn't create multi-page bvecs.

Second I don't think making it multipage bvec aware is all that hard,
and we should aim for doing the proper thing.