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SubjectRe: memory hotplug and force_remove
Hi Michal,

On Fri, Mar 31, 2017 at 10:30:17AM +0200, Michal Hocko wrote:
> [Fixed up email address of Toshimitsu - the email thread starts
> http://lkml.kernel.org/r/20170320192938.GA11363@dhcp22.suse.cz]
>
> On Tue 28-03-17 17:22:58, Rafael J. Wysocki wrote:
> > On Tuesday, March 28, 2017 09:58:08 AM Michal Hocko wrote:
> > > On Mon 20-03-17 22:24:42, Rafael J. Wysocki wrote:
> > > > On Monday, March 20, 2017 03:29:39 PM Michal Hocko wrote:
> > > > > Hi Rafael,
> > > >
> > > > Hi,
> > > >
> > > > > we have been chasing the following BUG() triggering during the memory
> > > > > hotremove (remove_memory):
> > > > > ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> > > > > check_memblock_offlined_cb);
> > > > > if (ret)
> > > > > BUG();
> > > > >
> > > > > and it took a while to learn that the issue is caused by
> > > > > /sys/firmware/acpi/hotplug/force_remove being enabled. I was really
> > > > > surprised to see such an option because at least for the memory hotplug
> > > > > it cannot work at all. Memory hotplug fails when the memory is still
> > > > > in use. Even if we do not BUG() here enforcing the hotplug operation
> > > > > will lead to problematic behavior later like crash or a silent memory
> > > > > corruption if the memory gets onlined back and reused by somebody else.
> > > > >
> > > > > I am wondering what was the motivation for introducing this behavior and
> > > > > whether there is a way to disallow it for memory hotplug. Or maybe drop
> > > > > it completely. What would break in such a case?
> > > >
> > > > Honestly, I don't remember from the top of my head and I haven't looked at
> > > > that code for several months.
> > > >
> > > > I need some time to recall that.
> > >
> > > Did you have any chance to look into this?
> >
> > Well, yes.
> >
> > It looks like that was added for some people who depended on the old behavior
> > at that time.
> >
> > I guess we can try to drop it and see what happpens. :-)
>
> OK, so what do you think about the following? It is based on the current
> linux-next and I have only compile tested it.
> ---
> >From 6c5ae594ce938a1ae9b9718958401682bfab3980 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Fri, 31 Mar 2017 10:08:41 +0200
> Subject: [PATCH] acpi: drop support for force_remove
>
> /sys/firmware/acpi/hotplug/force_remove was presumably added to support
> auto offlining in the past. This is, however, inherently dangerous for
> some hotplugable resources like memory. The memory offlining fails when
> the memory is still in use and cannot be dropped or migrated. If we
> ignore the failure we are basically allowing for subtle memory
> corruption or a crash.
>
> We have actually noticed the later while hitting BUG() during the memory
> hotremove (remove_memory):
> ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> check_memblock_offlined_cb);
> if (ret)
> BUG();
>
> it took us quite non-trivial time realize that the customer had
> force_remove enabled. Even if the BUG was removed here and we could
> propagate the error up the call chain it wouldn't help at all because
> then we would hit a crash or a memory corruption later and harder to
> debug. So force_remove is unfixable for the memory hotremove. We haven't
> checked other hotplugable resources to be prone to a similar problems.
>
> Remove the force_remove functionality because it is not fixable currently.
> Keep the sysfs file and report an error if somebody tries to enable it.
> Encourage users to report about the missing functionality and work with
> them with an alternative solution.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> Documentation/ABI/obsolete/sysfs-firmware-acpi | 8 ++++++++
> Documentation/ABI/testing/sysfs-firmware-acpi | 10 ----------
> drivers/acpi/internal.h | 2 --
> drivers/acpi/scan.c | 17 +++--------------
> drivers/acpi/sysfs.c | 9 +++++----
> 5 files changed, 16 insertions(+), 30 deletions(-)
> create mode 100644 Documentation/ABI/obsolete/sysfs-firmware-acpi
>
> diff --git a/Documentation/ABI/obsolete/sysfs-firmware-acpi b/Documentation/ABI/obsolete/sysfs-firmware-acpi
> new file mode 100644
> index 000000000000..6715a71bec3d
> --- /dev/null
> +++ b/Documentation/ABI/obsolete/sysfs-firmware-acpi
> @@ -0,0 +1,8 @@
> +What: /sys/firmware/acpi/hotplug/force_remove
> +Date: Mar 2017
> +Contact: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> +Description:
> + Since the force_remove is inherently broken and dangerous to
> + use for some hotplugable resources like memory (because ignoring
> + the offline failure might lead to memory corruption and crashes)
> + enabling this knob is not safe and thus unsupported.
> diff --git a/Documentation/ABI/testing/sysfs-firmware-acpi b/Documentation/ABI/testing/sysfs-firmware-acpi
> index c7fc72d4495c..613f42a9d5cd 100644
> --- a/Documentation/ABI/testing/sysfs-firmware-acpi
> +++ b/Documentation/ABI/testing/sysfs-firmware-acpi
> @@ -44,16 +44,6 @@ Contact: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> or 0 (unset). Attempts to write any other values to it will
> cause -EINVAL to be returned.
>
> -What: /sys/firmware/acpi/hotplug/force_remove
> -Date: May 2013
> -Contact: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> -Description:
> - The number in this file (0 or 1) determines whether (1) or not
> - (0) the ACPI subsystem will allow devices to be hot-removed even
> - if they cannot be put offline gracefully (from the kernel's
> - viewpoint). That number can be changed by writing a boolean
> - value to this file.
> -
> What: /sys/firmware/acpi/interrupts/
> Date: February 2008
> Contact: Len Brown <lenb@kernel.org>
> diff --git a/drivers/acpi/internal.h b/drivers/acpi/internal.h
> index f15900132912..66229ffa909b 100644
> --- a/drivers/acpi/internal.h
> +++ b/drivers/acpi/internal.h
> @@ -65,8 +65,6 @@ static inline void acpi_cmos_rtc_init(void) {}
> #endif
> int acpi_rev_override_setup(char *str);
>
> -extern bool acpi_force_hot_remove;
> -
> void acpi_sysfs_add_hotplug_profile(struct acpi_hotplug_profile *hotplug,
> const char *name);
> int acpi_scan_add_handler_with_hotplug(struct acpi_scan_handler *handler,
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index 192691880d55..a8d893fcedca 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -30,12 +30,6 @@ extern struct acpi_device *acpi_root;
>
> #define INVALID_ACPI_HANDLE ((acpi_handle)empty_zero_page)
>
> -/*
> - * If set, devices will be hot-removed even if they cannot be put offline
> - * gracefully (from the kernel's standpoint).
> - */
> -bool acpi_force_hot_remove;
> -
> static const char *dummy_hid = "device";
>
> static LIST_HEAD(acpi_dep_list);
> @@ -170,9 +164,6 @@ static acpi_status acpi_bus_offline(acpi_handle handle, u32 lvl, void *data,
> pn->put_online = false;
> }
> ret = device_offline(pn->dev);
> - if (acpi_force_hot_remove)
> - continue;
> -
> if (ret >= 0) {
> pn->put_online = !ret;
> } else {
> @@ -241,11 +232,10 @@ static int acpi_scan_try_to_offline(struct acpi_device *device)
> acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX,
> NULL, acpi_bus_offline, (void *)true,
> (void **)&errdev);
> - if (!errdev || acpi_force_hot_remove)
> + if (!errdev)
> acpi_bus_offline(handle, 0, (void *)true,
> (void **)&errdev);
> -
> - if (errdev && !acpi_force_hot_remove) {
> + else {
^^^^^^^^^^^^^
Here should still checks the parent's errdev state then rollback
parent/children to online state:

- if (errdev && !acpi_force_hot_remove) {
+ if (errdev) {

> dev_warn(errdev, "Offline failed.\n");
> acpi_bus_online(handle, 0, NULL, NULL);
> acpi_walk_namespace(ACPI_TYPE_ANY, handle,
[...snip]

Thanks a lot!
Joey Lee

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