Re: [PATCH v3 2/6] PM / Runtime: introduce pm_runtime_set[get]_memalloc_noio()

From: Ming Lei <ming.lei@canonical.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-kernel@vger.kernel.org, Oliver Neukum <oneukum@suse.de>,
	Minchan Kim <minchan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rjw@sisk.pl>, Jens Axboe <axboe@kernel.dk>,
	"David S. Miller" <davem@davemloft.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	netdev@vger.kernel.org, linux-usb@vger.kernel.org,
	linux-pm@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 2/6] PM / Runtime: introduce pm_runtime_set[get]_memalloc_noio()
Date: Wed, 31 Oct 2012 00:00:56 +0800	[thread overview]
Message-ID: <CACVXFVO5-UPNrWsySzDE5AfOv1TMqbyitQX9ViidSJPM36fqAQ@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1210301112270.1363-100000@iolanthe.rowland.org>

On Tue, Oct 30, 2012 at 11:38 PM, Alan Stern <stern@rowland.harvard.edu> wrote:

>
> Okay, I see your point.  But acquiring the lock here doesn't solve the
> problem.  Suppose a thread is about to reset a USB mass-storage device.
> It acquires the lock and sees that the noio flag is clear.  But before
> it can issue the reset, another thread sets the noio flag.

If the USB mass-storage device is being reseted, the flag should be set
already generally.  If the flag is still unset, that means the disk/network
device isn't added into system(or removed just now), so memory allocation
with block I/O should be allowed during the reset. Looks it isn't one problem,
isn't it?

> I'm not sure what the best solution is.
>
>> The lock needn't to be held when the function is called inside
>> pm_runtime_set_memalloc_noio(),  so the bitfield flag should
>> be checked directly without holding power lock in dev_memalloc_noio().
>
> Yes.
>
> A couple of other things...  Runtime resume can be blocked by runtime
> suspend, if a resume is requested while the suspend is in progress.
> Therefore the runtime suspend code also needs to save-set-restore the
> noio flag.

Looks the simplest approach is to handle the noio flag thing at the start and
end of rpm_resume.

> Also, we should set the noio flag at the start of
> usb_stor_control_thread, because everything that thread does can
> potentially block an I/O operation.

Yes, it should be done, and all GFP_NOIO in usbcore should be converted
into GFP_KERNEL together. And the work shouldn't be started until
the patchset is merged.

> Lastly, pm_runtime_get_memalloc_noio always returns false when
> CONFIG_PM_RUNTIME is disabled.  But we still need to prevent I/O during
> usb_reset_device even when there's no runtime PM.  Maybe the simplest
> answer is always to set noio during resets.  That would also help with
> the race described above.

I have thought about this. IMO, pm_runtime_get_memalloc_noio should
return true always if CONFIG_PM_RUNTIME is unset.

Thanks,
--
Ming Lei