Message-ID: <1354208592.26955.429.camel@misato.fc.hp.com>
Date:	Thu, 29 Nov 2012 10:03:12 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	linux-acpi@...r.kernel.org, Hanjun Guo <guohanjun@...wei.com>,
	Vasilis Liaskovitis <vasilis.liaskovitis@...fitbricks.com>,
	isimatu.yasuaki@...fujitsu.com, wency@...fujitsu.com,
	lenb@...nel.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tang Chen <tangchen@...fujitsu.com>
Subject: Re: [RFC PATCH v3 0/3] acpi: Introduce prepare_remove device
 operation

On Thu, 2012-11-29 at 11:15 +0100, Rafael J. Wysocki wrote:
> On Wednesday, November 28, 2012 11:41:36 AM Toshi Kani wrote:
> > On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:
> > > On 2012/11/24 1:50, Vasilis Liaskovitis wrote:
> > > > As discussed in https://patchwork.kernel.org/patch/1581581/
> > > > the driver core remove function needs to always succeed. This means we need
> > > > to know that the device can be successfully removed before acpi_bus_trim / 
> > > > acpi_bus_hot_remove_device are called. This can cause panics when OSPM-initiated
> > > > or SCI-initiated eject of memory devices fail e.g with:
> > > > echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
> > > > 
> > > > since the ACPI core goes ahead and ejects the device regardless of whether the
> > > > the memory is still in use or not.
> > > > 
> > > > For this reason a new acpi_device operation called prepare_remove is introduced.
> > > > This operation should be registered for acpi devices whose removal (from kernel
> > > > perspective) can fail.  Memory devices fall in this category.
> > > > 
> > > > acpi_bus_remove() is changed to handle removal in 2 steps:
> > > > - preparation for removal i.e. perform part of removal that can fail. Should
> > > >   succeed for device and all its children.
> > > > - if above step was successfull, proceed to actual device removal
> > > 
> > > Hi Vasilis,
> > > We met the same problem when we doing computer node hotplug, It is a good idea
> > > to introduce prepare_remove before actual device removal.
> > > 
> > > I think we could do more in prepare_remove, such as rollback. In most cases, we can
> > > offline most of memory sections except kernel used pages now, should we rollback
> > > and online the memory sections when prepare_remove failed ?
> > 
> > I think hot-plug operation should have all-or-nothing semantics.  That
> > is, an operation should either complete successfully, or rollback to the
> > original state.
> 
> That's correct.
> 
> > > As you may know, the ACPI based hotplug framework we are working on already addressed
> > > this problem, and the way we slove this problem is a bit like yours.
> > > 
> > > We introduce hp_ops in struct acpi_device_ops:
> > > struct acpi_device_ops {
> > > 	acpi_op_add add;
> > > 	acpi_op_remove remove;
> > > 	acpi_op_start start;
> > > 	acpi_op_bind bind;
> > > 	acpi_op_unbind unbind;
> > > 	acpi_op_notify notify;
> > > #ifdef	CONFIG_ACPI_HOTPLUG
> > > 	struct acpihp_dev_ops *hp_ops;
> > > #endif	/* CONFIG_ACPI_HOTPLUG */
> > > };
> > > 
> > > in hp_ops, we divide the prepare_remove into six small steps, that is:
> > > 1) pre_release(): optional step to mark device going to be removed/busy
> > > 2) release(): reclaim device from running system
> > > 3) post_release(): rollback if cancelled by user or error happened
> > > 4) pre_unconfigure(): optional step to solve possible dependency issue
> > > 5) unconfigure(): remove devices from running system
> > > 6) post_unconfigure(): free resources used by devices
> > > 
> > > In this way, we can easily rollback if error happens.
> > > How do you think of this solution, any suggestion ? I think we can achieve
> > > a better way for sharing ideas. :)
> > 
> > Yes, sharing idea is good. :)  I do not know if we need all 6 steps (I
> > have not looked at all your changes yet..), but in my mind, a hot-plug
> > operation should be composed with the following 3 phases.
> > 
> > 1. Validate phase - Verify if the request is a supported operation.  All
> > known restrictions are verified at this phase.  For instance, if a
> > hot-remove request involves kernel memory, it is failed in this phase.
> > Since this phase makes no change, no rollback is necessary to fail.  
> 
> Actually, we can't do it this way, because the conditions may change between
> the check and the execution.  So the first phase needs to involve execution
> to some extent, although only as far as it remains reversible.

For memory hot-remove, we can check if the target memory ranges are
within ZONE_MOVABLE.  We should not allow user to change this setup
during hot-remove operation.  Other things may be to check if a target
node contains cpu0 (until it is supported), the console UART (assuming
we cannot delete it), etc.  We should avoid doing rollback as much as we
can.

Thanks,
-Toshi


> > 2. Execute phase - Perform hot-add / hot-remove operation that can be
> > rolled-back in case of error or cancel.
> 
> I would just merge 1 and 2.
> 
> > 3. Commit phase - Perform the final hot-add / hot-remove operation that
> > cannot be rolled-back.  No error / cancel is allowed in this phase.  For
> > instance, eject operation is performed at this phase.  
> 
> Yup.
> 
> Thanks,
> Rafael
> 
> 


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