linux-kernel - Re: [RFC 2/5] fs: freeze on suspend and thaw on resume

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Message-ID: <20171003203936.GH2294@wotan.suse.de>
Date:   Tue, 3 Oct 2017 22:39:36 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "ONeukum@...e.com" <ONeukum@...e.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nborisov@...e.com" <nborisov@...e.com>,
        "oleg.b.antonyan@...il.com" <oleg.b.antonyan@...il.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "pavel@....cz" <pavel@....cz>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
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        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
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        "tytso@....edu" <tytso@....edu>, "jack@...e.cz" <jack@...e.cz>
Subject: Re: [RFC 2/5] fs: freeze on suspend and thaw on resume

On Tue, Oct 03, 2017 at 08:32:39PM +0000, Bart Van Assche wrote:
> On Tue, 2017-10-03 at 22:23 +0200, Luis R. Rodriguez wrote:
> > On Tue, Oct 03, 2017 at 08:02:22PM +0000, Bart Van Assche wrote:
> > > On Tue, 2017-10-03 at 11:53 -0700, Luis R. Rodriguez wrote:
> > > > +static bool super_allows_freeze(struct super_block *sb)
> > > > +{
> > > > +	return !!(sb->s_type->fs_flags & FS_FREEZE_ON_SUSPEND);
> > > > +}
> > > 
> > > A minor comment: if "!!" would be left out the compiler will perform the
> > > conversion from int to bool implicitly
> > 
> > For all compilers?
> 
> Let's have a look at the output of the following commands:
> 
> $ PAGER= git grep 'typedef.*[[:blank:]]bool;' include
> include/linux/types.h:typedef _Bool                     bool;
> $ PAGER= git grep std= Makefile
> Makefile:               -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)
> Makefile:                  -std=gnu89 $(call cc-option,-fno-PIE)
> 
> From https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/C-Dialect-Options.html#C-Dialect-Options:
> ‘gnu89’
> GNU dialect of ISO C90 (including some C99 features).
> 
> I think this means that the Linux kernel tree can only be compiled correctly
> by compilers that support the C11 type _Bool.

:*) beautiful, thanks.

> > > Anyway, I agree with the approach of this patch and I think
> > > that freezing filesystems before processes are frozen would be a big step
> > > forward.
> > 
> > Great! But please note, the current implementation calls fs_suspend_freeze()
> > *after* try_to_freeze_tasks(), ie: this implementation freezes userspace and
> > only after then filesystems.
> 
> What will the impact be of that choice on filesystems implemented in userspace?

Depends on what kernel hooks those use? Also now is a good time for those working
on userspace filesystems to chime in :) Its why I am stating -- I am not saying
I have found the right order, I have find the right order that works for me, and
we need consensus on what the right order should be.

  Luis