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

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Date:	Thu, 14 Feb 2013 11:31:53 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Pavel Machek <pavel@....cz>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Li Fei <fei.li@...el.com>, len.brown@...el.com,
	mingo@...hat.com, peterz@...radead.org, biao.wang@...el.com,
	linux-pm@...r.kernel.org, fuse-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, chuansheng.liu@...el.com
Subject: Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH]
 fuse: make fuse daemon frozen along with kernel threads]

On Wed, Feb 13, 2013 at 9:16 PM, Pavel Machek <pavel@....cz> wrote:
> On Wed 2013-02-13 18:34:16, Miklos Szeredi wrote:
>> On Tue, Feb 12, 2013 at 2:17 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>> > On Tue, Feb 12, 2013 at 2:13 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>> >> On Tue, Feb 12, 2013 at 11:46 AM, Pavel Machek <pavel@....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
>
> And it looked so easy and nice...
>
>> 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.
>
> Is it even valid for get_user_pages to be used on FUSE?

Disabling all mmap (which is what you question) would make fuse a
useless piece of toy.  So yes, definitely it's valid.

>
> What happens if fused tries to do some operation (it is userspace, it
> can do lot of operations) that needs one of those locks?

E.g. mmap_sem is taken for the process _accessing_ a vma for a fuse
filesystem. The server process better not do this (i.e. access the
filesystem it's serving) or deadlocking (*) is basically guaranteed.
That's nothing new.

The thing that makes this hard for freeze is that those locks
(mmap_sem, etc.) are only indirectly related to the fuse filesystem by
the _actions_ of a process (accessing mmaped fuse area).

Thanks,
Miklos

(*) userspace induced deadlocks in fuse are not hard deadlocks, they
can be forced open by "umount -f" or "echo >
/sys/fs/fuse/connections/$DEV/abort".
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