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

[David Hildenbrand <david@xxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  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.

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".

-- 
Thanks,

David / dhildenb