linux-kernel - Re: [PATCH v3 6/6] loop: increment sequence number

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Message-ID: <YNMhwLMr7DiNdqC/@infradead.org>
Date:   Wed, 23 Jun 2021 12:57:52 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, 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 Gonz??lez <javier@...igon.com>,
        Niklas Cassel <niklas.cassel@....com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Hannes Reinecke <hare@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        JeffleXu <jefflexu@...ux.alibaba.com>
Subject: Re: [PATCH v3 6/6] loop: increment sequence number

On Wed, Jun 23, 2021 at 12:58:58PM +0200, Matteo Croce wrote:
> From: Matteo Croce <mcroce@...rosoft.com>
> 
> On a very loaded system, if there are many events queued up from multiple
> attach/detach cycles, it's impossible to match them up with the
> LOOP_CONFIGURE or LOOP_SET_FD call, since we don't know where the position
> of our own association in the queue is[1].
> Not even an empty uevent queue is a reliable indication that we already
> received the uevent we were waiting for, since with multi-partition block
> devices each partition's event is queued asynchronously and might be
> delivered later.
> 
> Increment the disk sequence number when setting or changing the backing
> file, so the userspace knows which backing file generated the event:

Instead of manually incrementing the sequence here, can we make loop
generate the DISK_EVENT_MEDIA_CHANGE event on a backing device (aka
media) change?