linux-kernel - Re: [PATCH 2/5] mm/memory_hotplug: Create add/del_device

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Message-ID: <20181112191434.z5ufchwele3fl6se@soleen.tm1wkky2jk1uhgkn0ivaxijq1c.bx.internal.cloudapp.net>
Date:   Mon, 12 Nov 2018 19:14:34 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Oscar Salvador <osalvador@...hadventures.net>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, yasu.isimatu@...il.com,
        rppt@...ux.vnet.ibm.com, malat@...ian.org,
        linux-kernel@...r.kernel.org, pavel.tatashin@...rosoft.com,
        jglisse@...hat.com, Jonathan.Cameron@...wei.com, rafael@...nel.org,
        david@...hat.com, dave.jiang@...el.com, linux-mm@...ck.org,
        alexander.h.duyck@...ux.intel.com,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 2/5] mm/memory_hotplug: Create add/del_device_memory
 functions

On 18-10-15 17:30:31, Oscar Salvador wrote:
> From: Oscar Salvador <osalvador@...e.de>
> 
> HMM/devm have a particular handling of memory-hotplug.
> They do not go through the common path, and so, they do not
> call either offline_pages() or online_pages().
> 
> The operations they perform are the following ones:
> 
> 1) Create the linear mapping in case the memory is not private
> 2) Initialize the pages and add the sections
> 3) Move the pages to ZONE_DEVICE
> 
> Due to this particular handling of hot-add/remove memory from HMM/devm,
> I think it would be nice to provide a helper function in order to
> make this cleaner, and not populate other regions with code
> that should belong to memory-hotplug.
> 
> The helpers are named:
> 
> del_device_memory
> add_device_memory
> 
> The idea is that add_device_memory will be in charge of:
> 
> a) call either arch_add_memory() or add_pages(), depending on whether
>    we want a linear mapping
> b) online the memory sections that correspond to the pfn range
> c) call move_pfn_range_to_zone() being zone ZONE_DEVICE to
>    expand zone/pgdat spanned pages and initialize its pages
> 
> del_device_memory, on the other hand, will be in charge of:
> 
> a) offline the memory sections that correspond to the pfn range
> b) call shrink_zone_pgdat_pages(), which shrinks node/zone spanned pages.
> c) call either arch_remove_memory() or __remove_pages(), depending on
>    whether we need to tear down the linear mapping or not
> 
> The reason behind step b) from add_device_memory() and step a)
> from del_device_memory is that now find_smallest/biggest_section_pfn
> will have to check for online sections, and not for valid sections as
> they used to do, because we call offline_mem_sections() in
> offline_pages().
> 
> In order to split up better the patches and ease the review,
> this patch will only make a) case work for add_device_memory(),
> and case c) for del_device_memory.
> 
> The other cases will be added in the next patch.
> 
> These two functions have to be called from devm/HMM code:
> 
> dd_device_memory:
>         - devm_memremap_pages()
>         - hmm_devmem_pages_create()
> 
> del_device_memory:
>         - hmm_devmem_release
>         - devm_memremap_pages_release
> 
> One thing I do not know is whether we can move kasan calls out of the
> hotplug lock or not.
> If we can, we could move the hotplug lock within add/del_device_memory().
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>

Looks good to me, thank you.

Reviewed-by: Pavel Tatashin <pasha.tatashin@...een.com>

Pasha