On Thu, Apr 30, 2020 at 12:20 AM David Hildenbrand <david@xxxxxxxxxx> 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@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. 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 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.
