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

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Date:   Wed, 23 Jun 2021 14:13:25 +0100
From:   Luca Boccassi <bluca@...ian.org>
To:     Christoph Hellwig <hch@...radead.org>,
        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>,
        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>,
        JeffleXu <jefflexu@...ux.alibaba.com>
Subject: Re: [PATCH v3 6/6] loop: increment sequence number

On Wed, 2021-06-23 at 12:57 +0100, Christoph Hellwig wrote:
> 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?

Hi,

This was answered in the v1 thread:

https://lore.kernel.org/linux-fsdevel/20210315201331.GA2577561@casper.infradead.org/t/#m8a677028572e826352cbb1e19d1b9c1f3b6bff4b

The fundamental issue is that we'd be back at trying to correlate
events to loopdev instances, which does not work reliably - hence this
patch series. With the new ioctl, we can get the id immediately and
without delay when we create the device, with no possible races. Then
we can handle events reliably, as we can correlate correctly in all
cases.

-- 
Kind regards,
Luca Boccassi

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