linux-kernel - Re: [PATCH v2 2/3] dm: Export function dm_suspend

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Message-ID: <20150717152253.GA1021@redhat.com>
Date:	Fri, 17 Jul 2015 11:22:53 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Alasdair Kergon <agk@...hat.com>, Neil Brown <neilb@...e.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	linux-raid@...r.kernel.org, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/3] dm: Export function dm_suspend_md()

On Fri, Jul 17 2015 at 10:22am -0400,
Pali Rohár <pali.rohar@...il.com> wrote:

> On Friday 17 July 2015 10:04:39 Mike Snitzer wrote:
> > On Sun, Jun 21 2015 at  7:20am -0400,
> > Pali Rohár <pali.rohar@...il.com> wrote:
> > 
> > > This patch exports function dm_suspend_md() which suspend mapped device so other
> > > kernel drivers can use it and could suspend mapped device when needed.
> > > 
> > > Signed-off-by: Pali Rohár <pali.rohar@...il.com>
> > > ---
> > >  drivers/md/dm.c |    6 ++++++
> > >  drivers/md/dm.h |    5 +++++
> > >  2 files changed, 11 insertions(+)
> > > 
> > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > > index 2caf492..03298ff 100644
> > > --- a/drivers/md/dm.c
> > > +++ b/drivers/md/dm.c
> > > @@ -3343,6 +3343,12 @@ out:
> > >  	return r;
> > >  }
> > >  
> > > +int dm_suspend_md(struct mapped_device *md)
> > > +{
> > > +	return dm_suspend(md, DM_SUSPEND_LOCKFS_FLAG);
> > > +}
> > > +EXPORT_SYMBOL_GPL(dm_suspend_md);
> > > +
> > >  /*
> > >   * Internal suspend/resume works like userspace-driven suspend. It waits
> > >   * until all bios finish and prevents issuing new bios to the target drivers.
> > 
> > To do this properly you should be introducing a variant of
> > dm_internal_suspend.  We currently have two variants:
> > dm_internal_suspend_fast
> > dm_internal_suspend_noflush
> > 
> > The reason to use a dm_internal_suspend variant is this suspend was
> > _not_ initiated by an upper level ioctl (from userspace).  It was
> > done internally from within the target.
> > 
> > You're explicitly using DM_SUSPEND_LOCKFS_FLAG above.. meaning you're
> > interested in flushing all pending IO (in the FS layered on dm-crupt, if
> > one exists).
> > 
> > But see the comment in __dm_internal_suspend() about TASK_UNINTERRUPTIBLE.
> > If you're OK with the dm-crypt initiated suspend being
> > TASK_UNINTERRUPTIBLE then you could just introduce:
> > 
> > void dm_internal_suspend_uninterruptible_flush(struct mapped_device *md)
> > {
> >         mutex_lock(&md->suspend_lock);
> >         __dm_internal_suspend(md, DM_SUSPEND_LOCKFS_FLAG);
> >         mutex_unlock(&md->suspend_lock);
> > }
> > EXPORT_SYMBOL_GPL(dm_internal_suspend_uninterruptible_flush);
> > 
> > Otherwise, there is much more extensive DM core changes needed to
> > __dm_internal_suspend() and .presuspend to properly support
> > TASK_INTERRUPTIBLE.
> 
> Hi! I will look at dm_internal_suspend. Anyway use case for suspend is
> from dm-crypt to do both operations: suspend + key wipe. It means that
> without entering key again from userspace, resume is not possible. So my
> question is: It is possible to do internal suspend and then using resume
> from userspace via ioctl?

Good question: no, userspace resume would block waiting for internal
resume.

Soooo... I'll have to look at your patch 3 to understand why dm-crypt is
needing to initiate the suspend internally but then userspace is
initiating the resume... this imbalance is concerning.
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