From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Re: [systemd-devel] systemd kills mdmon if it was started manually
 by user
Date: Mon, 7 Nov 2011 15:30:37 +1100
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To: Kay Sievers <kay.sievers@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>, linux-raid@vger.kernel.org, Dan Williams <dan.j.williams@intel.com>, Andrey Borzenkov <arvidjaar@mail.ru>, systemd-devel@lists.freedesktop.org
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On Mon, 7 Nov 2011 04:42:54 +0100 Kay Sievers <kay.sievers@vrfy.org> wrote:

> On Mon, Nov 7, 2011 at 03:52, NeilBrown <neilb@suse.de> wrote:
>=20
> > However there is an important piece missing. =A0When you remount,ro a
> > filesystem, the block device doesn't get told so it thinks it is still =
open
> > read/write. =A0So md cannot tell mdmon that the array is now read-only
>=20
> That ro/rw flag is visible in /proc/self/mountinfo, shouldn't it be
> possible for mdmon to poll() that file and let the kernel wake stuff
> up when the ro/rw flag changes, like we do for the usual mount changes
> already?
>=20
> Kay

The ro/rw flag for file systems is in /proc/self/mountinfo.

However I want the ro/rw flag for the block device.
A block device can be partitioned so it might have multiple filesystems on =
it.
and it might have swap too.
or a dm table or another md device or an open file descriptor or ....

Yes, I could maybe parse various different files and try to work out what is
going on.  But the kernel can easily *know* what is going on.

Making this work "perfectly" would require md dropping its write-access to
member devices when the last write-access to the top level device goes.  And
the same for dm and loop and .....

But just filesystems would go a long way to catching the common cases
correctly.

Thanks,
NeilBrown


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