Date:   Wed, 28 Jul 2021 13:22:18 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matteo Croce <mcroce@...ux.microsoft.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        Luca Boccassi <bluca@...ian.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@....com>,
        Tejun Heo <tj@...nel.org>,
        Javier González <javier@...igon.com>,
        Niklas Cassel <niklas.cassel@....com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Hannes Reinecke <hare@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        JeffleXu <jefflexu@...ux.alibaba.com>
Subject: Re: [PATCH v5 0/5] block: add a sequence number to disks

On 7/12/21 5:05 PM, Matteo Croce wrote:
> From: Matteo Croce <mcroce@...rosoft.com>
> 
> Associating uevents with block devices in userspace is difficult and racy:
> the uevent netlink socket is lossy, and on slow and overloaded systems has
> a very high latency. Block devices do not have exclusive owners in
> userspace, any process can set one up (e.g. loop devices). Moreover, device
> names can be reused (e.g. loop0 can be reused again and again). A userspace
> process setting up a block device and watching for its events cannot thus
> reliably tell whether an event relates to the device it just set up or
> another earlier instance with the same name.
> 
> Being able to set a UUID on a loop device would solve the race conditions.
> But it does not allow to derive orderings from uevents: if you see a uevent
> with a UUID that does not match the device you are waiting for, you cannot
> tell whether it's because the right uevent has not arrived yet, or it was
> already sent and you missed it. So you cannot tell whether you should wait
> for it or not.
> 
> Being able to set devices up in a namespace would solve the race conditions
> too, but it can work only if being namespaced is feasible in the first
> place. Many userspace processes need to set devices up for the root
> namespace, so this solution cannot always work.
> 
> Changing the loop devices naming implementation to always use
> monotonically increasing device numbers, instead of reusing the lowest
> free number, would also solve the problem, but it would be very disruptive
> to userspace and likely break many existing use cases. It would also be
> quite awkward to use on long-running machines, as the loop device name
> would quickly grow to many-digits length.
> 
> Furthermore, this problem does not affect only loop devices - partition
> probing is asynchronous and very slow on busy systems. It is very easy to
> enter races when using LO_FLAGS_PARTSCAN and watching for the partitions to
> show up, as it can take a long time for the uevents to be delivered after
> setting them up.
> 
> Associating a unique, monotonically increasing sequential number to the
> lifetime of each block device, which can be retrieved with an ioctl
> immediately upon setting it up, allows to solve the race conditions with
> uevents, and also allows userspace processes to know whether they should
> wait for the uevent they need or if it was dropped and thus they should
> move on.
> 
> This does not benefit only loop devices and block devices with multiple
> partitions, but for example also removable media such as USB sticks or
> cdroms/dvdroms/etc.
> 
> The first patch is the core one, the 2..4 expose the information in
> different ways, and the last one makes the loop device generate a media
> changed event upon attach, detach or reconfigure, so the sequence number
> is increased.
> 
> If merged, this feature will immediately used by the userspace:
> https://github.com/systemd/systemd/issues/17469#issuecomment-762919781

Applied for 5.15, with #2 done manually since it didn't apply cleanly.

-- 
Jens Axboe

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