Re: [PATCH v1 2/3] mm/memory_hotplug: Introduce MHP_DRIVER_MANAGED

David Hildenbrand
2020-04-30T08:20:37Z

On 30.04.20 10:11, Dan Williams wrote:
> On Thu, Apr 30, 2020 at 12:20 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 29.04.20 18:08, David Hildenbrand wrote:
>>> Some paravirtualized devices that add memory via add_memory() and
>>> friends (esp. virtio-mem) don't want to create entries in
>>> /sys/firmware/memmap/ - primarily to hinder kexec from adding this
>>> memory to the boot memmap of the kexec kernel.
>>>
>>> In fact, such memory is never exposed via the firmware (e.g., e820), but
>>> only via the device, so exposing this memory via /sys/firmware/memmap/ is
>>> wrong:
>>> "kexec needs the raw firmware-provided memory map to setup the
>>> parameter segment of the kernel that should be booted with
>>> kexec. Also, the raw memory map is useful for debugging. For
>>> that reason, /sys/firmware/memmap is an interface that provides
>>> the raw memory map to userspace." [1]
>>>
>>> We want to let user space know that memory which is always detected,
>>> added, and managed via a (device) driver - like memory managed by
>>> virtio-mem - is special. It cannot be used for placing kexec segments
>>> and the (device) driver is responsible for re-adding memory that
>>> (eventually shrunk/grown/defragmented) memory after a reboot/kexec. It
>>> should e.g., not be added to a fixed up firmware memmap. However, it should
>>> be dumped by kdump.
>>>
>>> Also, such memory could behave differently than an ordinary DIMM - e.g.,
>>> memory managed by virtio-mem can have holes inside added memory resource,
>>> which should not be touched, especially for writing.
>>>
>>> Let's expose that memory as "System RAM (driver managed)" e.g., via
>>> /pro/iomem.
>>>
>>> We don't have to worry about firmware_map_remove() on the removal path.
>>> If there is no entry, it will simply return with -EINVAL.
>>>
>>> [1] https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-firmware-memmap
>>>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
>>> Cc: Wei Yang <richard.weiyang@gmail.com>
>>> Cc: Baoquan He <bhe@redhat.com>
>>> Cc: Eric Biederman <ebiederm@xmission.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>> include/linux/memory_hotplug.h | 8 ++++++++
>>> mm/memory_hotplug.c | 20 ++++++++++++++++----
>>> 2 files changed, 24 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>>> index bf0e3edb8688..cc538584b39e 100644
>>> --- a/include/linux/memory_hotplug.h
>>> +++ b/include/linux/memory_hotplug.h
>>> @@ -68,6 +68,14 @@ struct mhp_params {
>>> pgprot_t pgprot;
>>> };
>>>
>>> +/* Flags used for add_memory() and friends. */
>>> +
>>> +/*
>>> + * Don't create entries in /sys/firmware/memmap/ and expose memory as
>>> + * "System RAM (driver managed)" in e.g., /proc/iomem
>>> + */
>>> +#define MHP_DRIVER_MANAGED 1
>>> +
>>> /*
>>> * Zone resizing functions
>>> *
>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>> index ebdf6541d074..cfa0721280aa 100644
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -98,11 +98,11 @@ void mem_hotplug_done(void)
>>> u64 max_mem_size = U64_MAX;
>>>
>>> /* add this memory to iomem resource */
>>> -static struct resource *register_memory_resource(u64 start, u64 size)
>>> +static struct resource *register_memory_resource(u64 start, u64 size,
>>> + const char *resource_name)
>>> {
>>> struct resource *res;
>>> unsigned long flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
>>> - char *resource_name = "System RAM";
>>>
>>> /*
>>> * Make sure value parsed from 'mem=' only restricts memory adding
>>> @@ -1058,7 +1058,8 @@ int __ref add_memory_resource(int nid, struct resource *res,
>>> BUG_ON(ret);
>>>
>>> /* create new memmap entry */
>>> - firmware_map_add_hotplug(start, start + size, "System RAM");
>>> + if (!(flags & MHP_DRIVER_MANAGED))
>>> + firmware_map_add_hotplug(start, start + size, "System RAM");
>>>
>>> /* device_online() will take the lock when calling online_pages() */
>>> mem_hotplug_done();
>>> @@ -1081,10 +1082,21 @@ int __ref add_memory_resource(int nid, struct resource *res,
>>> /* requires device_hotplug_lock, see add_memory_resource() */
>>> int __ref __add_memory(int nid, u64 start, u64 size, unsigned long flags)
>>> {
>>> + const char *resource_name = "System RAM";
>>> struct resource *res;
>>> int ret;
>>>
>>> - res = register_memory_resource(start, size);
>>> + /*
>>> + * Indicate that memory managed by a driver is special. It's always
>>> + * detected and added via a driver, should not be given to the kexec
>>> + * kernel for booting when manually crafting the firmware memmap, and
>>> + * no kexec segments should be placed on it. However, kdump should
>>> + * dump this memory.
>>> + */
>>> + if (flags & MHP_DRIVER_MANAGED)
>>> + resource_name = "System RAM (driver managed)";
>>> +
>>> + res = register_memory_resource(start, size, resource_name);
>>> if (IS_ERR(res))
>>> return PTR_ERR(res);
>>>
>>>
>>
>> BTW, I was wondering if this is actually also something that
>> drivers/dax/kmem.c wants to use for adding memory.
>>
>> Just because we decided to use some DAX memory in the current kernel as
>> system ram, doesn't mean we should make that decision for the kexec
>> kernel (e.g., using it as initial memory, placing kexec binaries onto
>> it, etc.). This is also not what we would observe during a real reboot.
>
> Agree.
>
>> I can see that the "System RAM" resource will show up as child resource
>> under the device e.g., in /proc/iomem.
>>
>> However, entries in /sys/firmware/memmap/ are created as "System RAM".
>
> True. Do you think this rename should just be limited to what type
> /sys/firmware/memmap/ emits? I have the concern, but no proof

We could split this patch into

MHP_NO_FIRMWARE_MEMMAP (create firmware memmap entries)

and

MHP_DRIVER_MANAGED (name of the resource)

See below, the latter might not be needed.

> currently, that there are /proc/iomem walkers that explicitly look for
> "System RAM", but might be thrown off by "System RAM (driver
> managed)". I was not aware of /sys/firmware/memmap until about 5
> minutes ago.

The only two users of /proc/iomem I am aware of are kexec-tools and some
s390x tools.

kexec-tools on x86-64 uses /sys/firmware/memmap to craft the initial
memmap, but uses /proc/iomem to
a) Find places for kexec images
b) Detect memory regions to dump via kdump

I am not yet sure if we really need the "System RAM (driver managed)"
part. If we can teach kexec-tools to
a) Don't place kexec images on "System RAM" that has a parent resource
(most likely requires kexec-tools changes)
b) Consider for kdump "System RAM" that has a parent resource
we might be able to avoid renaming that. (I assume that's already done)

E.g., regarding virtio-mem (patch #3) I am currently also looking into
creating a parent resource instead, like dax/kmem to avoid the rename:

:/# cat /proc/iomem
00000000-00000fff : Reserved
[...]
100000000-13fffffff : System RAM
140000000-33fffffff : virtio0
140000000-147ffffff : System RAM
148000000-14fffffff : System RAM
150000000-157ffffff : System RAM
340000000-303fffffff : virtio1
340000000-347ffffff : System RAM
3280000000-32ffffffff : PCI Bus 0000:00

--
Thanks,

David / dhildenb