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From:   Mike Snitzer <snitzer@redhat.com>
Date:   Thu, 22 Oct 2020 14:35:02 -0400
Message-ID: <CAMM=eLfTf2f2Me7f5tpL5DEGgKsqaFaAS0qTDVpLAYrwR5Jf5g@mail.gmail.com>
Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module
To:     Sergei Shtepa <sergei.shtepa@veeam.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        linux-xfs@vger.kernel.org,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>, rjw@rjwysocki.net,
        len.brown@intel.com, Pavel Machek <pavel@ucw.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        johannes.thumshirn@wdc.com, Ming Lei <ming.lei@redhat.com>,
        Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
        gustavo@embeddedor.com, Bart Van Assche <bvanassche@acm.org>,
        osandov@fb.com, koct9i@gmail.com,
        Damien Le Moal <damien.lemoal@wdc.com>, steve@sk2.org,
        linux-block <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-pm@vger.kernel.org, linux-mm@kvack.org,
        device-mapper development <dm-devel@redhat.com>
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On Wed, Oct 21, 2020 at 5:04 AM Sergei Shtepa <sergei.shtepa@veeam.com> wrote:
>
> Hello everyone! Requesting for your comments and suggestions.
>
> # blk-filter
>
> Block layer filter allows to intercept BIO requests to a block device.
>
> Interception is performed at the very beginning of the BIO request
> processing, and therefore does not affect the operation of the request
> processing queue. This also makes it possible to intercept requests from
> a specific block device, rather than from the entire disk.
>
> The logic of the submit_bio function has been changed - since the
> function execution results are not processed anywhere (except for swap
> and direct-io) the function won't return a value anymore.

Your desire to switch to a void return comes exactly when I've noticed
we need it.

->submit_bio's blk_qc_t return is the cookie assigned by blk-mq.  Up
to this point we haven't actually used it for bio-based devices but it
seems clear we'll soon need for bio-based IO polling support.

Just today, I've been auditing drivers/md/dm.c with an eye toward
properly handling the blk_qc_t return (or lack thereof) from various
DM methods.

It could easily be that __submit_bio_noacct and __submit_bio_noacct_mq
will be updated to do something meaningful with the returned cookie
(or that DM will) to facilitate proper IO polling.

Mike