linux-kernel - Re: update kernel-doc for MEMBLOCK_RSRV

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Message-ID: <7ffd0abd-27a1-40a8-b538-9a01e21abb29@redhat.com>
Date: Mon, 25 Aug 2025 20:32:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Mika Penttilä <mpenttil@...hat.com>,
 linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
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 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
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Subject: Re: update kernel-doc for MEMBLOCK_RSRV_NOINIT

On 25.08.25 18:58, Mike Rapoport wrote:
> On Mon, Aug 25, 2025 at 06:23:48PM +0200, David Hildenbrand wrote:
>>
>> I don't quite understand the interaction with PG_Reserved and why anybody
>> using this function should care.
>>
>> So maybe you can rephrase in a way that is easier to digest, and rather
>> focuses on what callers of this function are supposed to do vs. have the
>> liberty of not doing?
> 
> How about
>   
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index b96746376e17..fcda8481de9a 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
>    * via a driver, and never indicated in the firmware-provided memory map as
>    * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
>    * kernel resource tree.
> - * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
> - * not initialized (only for reserved regions).
> + * @MEMBLOCK_RSRV_NOINIT: reserved memory region for which struct pages are not
> + * fully initialized. Users of this flag are responsible to properly initialize
> + * struct pages of this region
>    * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
>    * either explictitly with memblock_reserve_kern() or via memblock
>    * allocation APIs. All memblock allocations set this flag.
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 154f1d73b61f..46b411fb3630 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1091,13 +1091,20 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>   
>   /**
>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
> - * for this region.
> + * MEMBLOCK_RSRV_NOINIT
> + *
>    * @base: the base phys addr of the region
>    * @size: the size of the region
>    *
> - * struct pages will not be initialized for reserved memory regions marked with
> - * %MEMBLOCK_RSRV_NOINIT.
> + * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
> + * not be fully initialized to allow the caller optimize their initialization.
> + *
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flag
> + * completely bypasses the initialization of struct pages for such region.
> + *
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled, struct pages in this
> + * region will be initialized with default values but won't be marked as
> + * reserved.

Sounds good.

I am surprised regarding "reserved", but I guess that's because we don't 
end up calling "reserve_bootmem_region()" on these regions in 
memmap_init_reserved_pages().

-- 
Cheers

David / dhildenb