Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads]

From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Pavel Machek <pavel@ucw.cz>,
	Goswin von Brederlow <goswin-v-b@web.de>,
	Li Fei <fei.li@intel.com>,
	len.brown@intel.com, mingo@redhat.com, peterz@infradead.org,
	biao.wang@intel.com, linux-pm@vger.kernel.org,
	fuse-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org,
	chuansheng.liu@intel.com
Subject: Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads]
Date: Wed, 13 Feb 2013 22:21:11 +0100	[thread overview]
Message-ID: <24327774.p6haZy6gvA@vostro.rjw.lan> (raw)
In-Reply-To: <CAJfpegsFThkzv3uF60AMXO7Y-yEzU_20HeGyPra0O8Vk5OqCGQ@mail.gmail.com>

On Wednesday, February 13, 2013 06:34:16 PM Miklos Szeredi wrote:
> On Tue, Feb 12, 2013 at 2:17 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
> > On Tue, Feb 12, 2013 at 2:13 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >> On Tue, Feb 12, 2013 at 11:46 AM, Pavel Machek <pavel@ucw.cz> wrote:
> >>
> >>> (After all, with FUSE, filesystem clients are just doing IPC. In ideal
> >>> world, that would be freezeable and killable with -9).
> 
> Well, I thought this can be constrained to locks directly related to
> the fuse filesystem itself, but it can't...  The reason is page
> faults.  Pretty much everyone and their brother uses get_user_pages*,
> holding various locks (mmap_sem for sure but others as well).  A fuse
> filesystem frozen during a page read will block those.
> 
> Separating those parts of the kernel that can be frozen from those
> that can't looks increasingly difficult.
> 
> So I think the PF_FREEZE_DAEMON idea (the patch from Li Fei that
> started this thread) may still be our best bet at handling this
> situation.  The idea being that pure "originator" processes (ones that
> take no part in serving filesystem syscalls) can be frozen up-front.
> Then the "fuse daemon" (or "server") processes are hopefully in a
> quiescent state and can be frozen without difficulty.
> 
> Unfortunately it needs help from userspace: the kernel can't easily
> guess which processes are part of a "fuse daemon" and which aren't.
> Fortunately we have a standard library (libfuse) that can tell it to
> the kernel for the vast majority of cases.

So basically the idea would be to introduce something like PF_FREEZE_LATE
for user space processes that need to be frozen after all of the other
(non-PF_FREEZE_LATE) user space processes have been frozen and hack fuse
to use that flag?

Rafael

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.