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SubjectRe: [v5 0/3] "Hotremove" persistent memory
> Hi Pavel,
>
> I am working on adding this sort of a workflow into a new daxctl command
> (daxctl-reconfigure-device)- this will allow changing the 'mode' of a
> dax device to kmem, online the resulting memory, and with your patches,
> also attempt to offline the memory, and change back to device-dax.
>
> In running with these patches, and testing the offlining part, I ran
> into the following lockdep below.
>
> This is with just these three patches on top of -rc7.
>
>
> [ +0.004886] ======================================================
> [ +0.001576] WARNING: possible circular locking dependency detected
> [ +0.001506] 5.1.0-rc7+ #13 Tainted: G O
> [ +0.000929] ------------------------------------------------------
> [ +0.000708] daxctl/22950 is trying to acquire lock:
> [ +0.000548] 00000000f4d397f7 (kn->count#424){++++}, at: kernfs_remove_by_name_ns+0x40/0x80
> [ +0.000922]
> but task is already holding lock:
> [ +0.000657] 000000002aa52a9f (mem_sysfs_mutex){+.+.}, at: unregister_memory_section+0x22/0xa0

I have studied this issue, and now have a clear understanding why it
happens, I am not yet sure how to fix it, so suggestions are welcomed
:)

Here is the problem:

When we offline pages we have the following call stack:

# echo offline > /sys/devices/system/memory/memory8/state
ksys_write
vfs_write
__vfs_write
kernfs_fop_write
kernfs_get_active
lock_acquire kn->count#122 (lock for
"memory8/state" kn)
sysfs_kf_write
dev_attr_store
state_store
device_offline
memory_subsys_offline
memory_block_action
offline_pages
__offline_pages
percpu_down_write
down_write
lock_acquire mem_hotplug_lock.rw_sem

When we unbind dax0.0 we have the following stack:
# echo dax0.0 > /sys/bus/dax/drivers/kmem/unbind
drv_attr_store
unbind_store
device_driver_detach
device_release_driver_internal
dev_dax_kmem_remove
remove_memory device_hotplug_lock
try_remove_memory mem_hotplug_lock.rw_sem
arch_remove_memory
__remove_pages
__remove_section
unregister_memory_section
remove_memory_section mem_sysfs_mutex
unregister_memory
device_unregister
device_del
device_remove_attrs
sysfs_remove_groups
sysfs_remove_group
remove_files
kernfs_remove_by_name
kernfs_remove_by_name_ns
__kernfs_remove kn->count#122

So, lockdep found the ordering issue with the above two stacks:

1. kn->count#122 -> mem_hotplug_lock.rw_sem
2. mem_hotplug_lock.rw_sem -> kn->count#122

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