linux-kernel - Re: [PATCH v2 1/2] memory-hotplug.rst: remove locking details from admin-guide

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Date:   Tue, 8 Jun 2021 16:01:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 1/2] memory-hotplug.rst: remove locking details from
 admin-guide

On Tue 08-06-21 15:38:54, David Hildenbrand wrote:
> We have the same content at Documentation/core-api/memory-hotplug.rst
> and it doesn't fit into the admin-guide. The documentation was
> accidentially duplicated when merging.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Muchun Song <songmuchun@...edance.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> Cc: linux-doc@...r.kernel.org
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>
Thanks!

> ---
>  .../admin-guide/mm/memory-hotplug.rst         | 39 -------------------
>  1 file changed, 39 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index c6bae2d77160..a783cf7c8e4c 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -415,45 +415,6 @@ Need more implementation yet....
>   - 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
>  ===========
>  
> -- 
> 2.31.1

-- 
Michal Hocko
SUSE Labs