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Date:   Thu, 16 Jun 2022 09:30:33 +0200
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To:     Oscar Salvador <osalvador@suse.de>
Cc:     Muchun Song <songmuchun@bytedance.com>, corbet@lwn.net,
        akpm@linux-foundation.org, paulmck@kernel.org,
        mike.kravetz@oracle.com, linux-doc@vger.kernel.org,
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        duanxiongchun@bytedance.com, smuchun@gmail.com
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v2 2/2] mm: memory_hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP
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On 16.06.22 05:57, Oscar Salvador wrote:
> On Wed, Jun 15, 2022 at 11:51:49AM +0200, David Hildenbrand wrote:
>> An easier/future-proof approach might simply be flagging the vmemmap
>> pages as being special. We reuse page flags for that, which don't have
>> semantics yet (i.e., PG_reserved indicates a boot-time allocation via
>> memblock).
> 
> The first versions of memmap_on_memory [1] introduced a new page type
> to represent such pages, not sure if that is what you mean, e.g:
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f91cb8898ff0..75f302a532f9 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -708,6 +708,7 @@  PAGEFLAG_FALSE(DoubleMap)
>  #define PG_kmemcg	0x00000200
>  #define PG_table	0x00000400
>  #define PG_guard	0x00000800
> +#define PG_vmemmap     0x00001000
>  
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> @@ -764,6 +765,24 @@  PAGE_TYPE_OPS(Table, table)
>   */
>  PAGE_TYPE_OPS(Guard, guard)
>  
> +/*
> + * Vmemmap pages refers to those pages that are used to create the memmap
> + * array, and reside within the same memory range that was hotppluged, so
> + * they are self-hosted. (see include/linux/memory_hotplug.h)
> + */
> +PAGE_TYPE_OPS(Vmemmap, vmemmap)
> +static __always_inline void SetPageVmemmap(struct page *page)
> +{
> +	__SetPageVmemmap(page);
> +	__SetPageReserved(page);
> +}
> +
> +static __always_inline void ClearPageVmemmap(struct page *page)
> +{
> +	__ClearPageVmemmap(page);
> +	__ClearPageReserved(page);
> +}
> 
> [1] https://patchwork.kernel.org/project/linux-mm/patch/20190725160207.19579-3-osalvador@suse.de/

IIRC, that was used to skip these patches on the offlining path before
we provided the ranges to offline_pages().

I'd not mess with PG_reserved, and give them a clearer name, to not
confuse them with other, ordinary, vmemmap pages that are not
self-hosted (maybe in the future we might want to flag all vmemmap pages
with a new type?).

I'd just try reusing the flag PG_owner_priv_1. And eventually, flag all
(v)memmap pages with a type PG_memmap. However, the latter would be
optional and might not be strictly required


So what think could make sense is

/* vmemmap pages that are self-hosted and cannot be optimized/freed. */
PG_vmemmap_self_hosted = PG_owner_priv_1,

-- 
Thanks,

David / dhildenb