Re: [v5 0/3] "Hotremove" persistent memory

[Dan Williams]

On Wed, May 15, 2019 at 11:12 AM Pavel Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> > 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
> :)

I would think that ACPI hotplug would have a similar problem, but it does this:

                acpi_unbind_memory_blocks(info);
                __remove_memory(nid, info->start_addr, info->length);

I wonder if that ordering prevents going too deep into the
device_unregister() call stack that you highlighted below.


>
> 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