linux-kernel - Re: [PATCH 1/3] mm,memory_hotplug: export mhp min alignment

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Date:   Wed, 2 Jun 2021 14:14:48 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Max Gurtovoy <mgurtovoy@...dia.com>,
        linux-nvme@...ts.infradead.org, dan.j.williams@...el.com,
        logang@...tatee.com, linux-mm@...ck.org, hch@....de
Cc:     sagi@...mberg.me, oren@...dia.com, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH 1/3] mm,memory_hotplug: export mhp min alignment

On 02.06.21 13:10, Max Gurtovoy wrote:
> Hotplugged memory has alignmet restrictions. E.g, it disallows all
> operations smaller than a sub-section and only allow operations smaller
> than a section for SPARSEMEM_VMEMMAP. Export the alignment restrictions
> for mhp users.
> 
> Signed-off-by: Max Gurtovoy <mgurtovoy@...dia.com>
> ---
>   include/linux/memory_hotplug.h |  5 +++++
>   mm/memory_hotplug.c            | 33 +++++++++++++++++++--------------
>   2 files changed, 24 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 28f32fd00fe9..c55a9049b11e 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -76,6 +76,7 @@ struct mhp_params {
>   
>   bool mhp_range_allowed(u64 start, u64 size, bool need_mapping);
>   struct range mhp_get_pluggable_range(bool need_mapping);
> +unsigned long mhp_get_min_align(void);
>   
>   /*
>    * Zone resizing functions
> @@ -248,6 +249,10 @@ void mem_hotplug_done(void);
>   	___page;				\
>    })
>   
> +static inline unsigned long mhp_get_min_align(void)
> +{
> +	return 0;
> +}
>   static inline unsigned zone_span_seqbegin(struct zone *zone)
>   {
>   	return 0;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 9e86e9ee0a10..161bb6704a9b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -270,24 +270,29 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
>   }
>   #endif /* CONFIG_HAVE_BOOTMEM_INFO_NODE */
>   
> +/*
> + * Disallow all operations smaller than a sub-section and only
> + * allow operations smaller than a section for
> + * SPARSEMEM_VMEMMAP. Note that check_hotplug_memory_range()
> + * enforces a larger memory_block_size_bytes() granularity for
> + * memory that will be marked online, so this check should only
> + * fire for direct arch_{add,remove}_memory() users outside of
> + * add_memory_resource().
> + */
> +unsigned long mhp_get_min_align(void)
> +{
> +	if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> +		return PAGES_PER_SUBSECTION;
> +	return PAGES_PER_SECTION;
> +}
> +EXPORT_SYMBOL_GPL(mhp_get_min_align);

We have to main interfaces to "hotplug" memory:

a) add_memory() and friends for System RAM, which have memory block 
alignment requirements.

b) memremap_pages(), which has the alignemnt requirements you mention here.

I feel like what you need would better be exposed in mm/memremap.c, for 
example, via "memremap_min_alignment" so it matches the "memremap_pages" 
semantics.

And then, memremap_pages() is only available with CONFIG_ZONE_DEVICE, 
which depends on SPARSEMEM_VMEMMAP. So you'll always have 
PAGES_PER_SUBSECTION.

I can already spot "memremap_compat_align", maybe you can reuse that or 
handle it accordingly in there?


-- 
Thanks,

David / dhildenb