linux-kernel - Re: [PATCH] docs: reorder memory-hotplug documentation

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Message-ID: <43092504-a95f-374d-f3db-b961dd8ac428@redhat.com>
Date:   Tue, 21 May 2019 12:41:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Jonathan Corbet <corbet@....net>
Cc:     linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: reorder memory-hotplug documentation

On 14.05.19 10:23, Mike Rapoport wrote:
> The "Locking Internals" section of the memory-hotplug documentation is
> duplicated in admin-guide and core-api. Drop the admin-guide copy as
> locking internals does not belong there.
> 
> While on it, move the "Future Work" section to the core-api part.

Looks sane, but the future work part is really outdated, can we remove
this completely?

> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>  Documentation/admin-guide/mm/memory-hotplug.rst | 51 -------------------------
>  Documentation/core-api/memory-hotplug.rst       | 11 ++++++
>  2 files changed, 11 insertions(+), 51 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index 5c4432c..72090ba 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -391,54 +391,3 @@ Physical memory remove
>  Need more implementation yet....
>   - Notification completion of remove works by OS to firmware.
>   - Guard from remove if not yet.
> -
> -
> -Locking Internals
> -=================
> -
> -When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
> -the device_hotplug_lock should be held to:
> -
> -- synchronize against online/offline requests (e.g. via sysfs). This way, memory
> -  block devices can only be accessed (.online/.state attributes) by user
> -  space once memory has been fully added. And when removing memory, we
> -  know nobody is in critical sections.
> -- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
> -
> -Especially, there is a possible lock inversion that is avoided using
> -device_hotplug_lock when adding memory and user space tries to online that
> -memory faster than expected:
> -
> -- device_online() will first take the device_lock(), followed by
> -  mem_hotplug_lock
> -- add_memory_resource() will first take the mem_hotplug_lock, followed by
> -  the device_lock() (while creating the devices, during bus_add_device()).
> -
> -As the device is visible to user space before taking the device_lock(), this
> -can result in a lock inversion.
> -
> -onlining/offlining of memory should be done via device_online()/
> -device_offline() - to make sure it is properly synchronized to actions
> -via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
> -
> -When adding/removing/onlining/offlining memory or adding/removing
> -heterogeneous/device memory, we should always hold the mem_hotplug_lock in
> -write mode to serialise memory hotplug (e.g. access to global/zone
> -variables).
> -
> -In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
> -mode allows for a quite efficient get_online_mems/put_online_mems
> -implementation, so code accessing memory can protect from that memory
> -vanishing.
> -
> -
> -Future Work
> -===========
> -
> -  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
> -    sysctl or new control file.
> -  - showing memory block and physical device relationship.
> -  - test and make it better memory offlining.
> -  - support HugeTLB page migration and offlining.
> -  - memmap removing at memory offline.
> -  - physical remove memory.
> diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
> index de7467e..e08be1c 100644
> --- a/Documentation/core-api/memory-hotplug.rst
> +++ b/Documentation/core-api/memory-hotplug.rst
> @@ -123,3 +123,14 @@ In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
>  mode allows for a quite efficient get_online_mems/put_online_mems
>  implementation, so code accessing memory can protect from that memory
>  vanishing.
> +
> +Future Work
> +===========
> +
> +  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
> +    sysctl or new control file.

... that already works if I am not completely missing the point here

> +  - showing memory block and physical device relationship.

... that is available for s390x only AFAIK

> +  - test and make it better memory offlining.

... no big news ;)

> +  - support HugeTLB page migration and offlining.

... I remember that Oscar was doing something in that area, Oscar?

> +  - memmap removing at memory offline.

... no, we don't want this. However, we should properly clean up zone
information when offlining

> +  - physical remove memory.

... I don't even understand what that means.


I'd vote for removing the future work part, this is pretty outdated.


-- 

Thanks,

David / dhildenb