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Date:   Tue, 7 Jun 2022 14:21:55 +0200
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To:     Wupeng Ma <mawupeng1@huawei.com>, corbet@lwn.net, will@kernel.org,
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References: <20220607093805.1354256-1-mawupeng1@huawei.com>
 <20220607093805.1354256-6-mawupeng1@huawei.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v3 5/6] mm: Add mirror flag back on initrd memory
In-Reply-To: <20220607093805.1354256-6-mawupeng1@huawei.com>
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On 07.06.22 11:38, Wupeng Ma wrote:
> From: Ma Wupeng <mawupeng1@huawei.com>
> 
> Initrd memory will be removed and then added in arm64_memblock_init() and this
> will cause it to lose all of its memblock flags. The lost of MEMBLOCK_MIRROR
> flag will lead to error log printed by find_zone_movable_pfns_for_nodes if
> the lower 4G range has some non-mirrored memory.
> 
> In order to solve this problem, the lost MEMBLOCK_MIRROR flag will be
> reinstalled if the origin memblock has this flag.
> 
> Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
> ---
>  arch/arm64/mm/init.c     |  9 +++++++++
>  include/linux/memblock.h |  1 +
>  mm/memblock.c            | 20 ++++++++++++++++++++
>  3 files changed, 30 insertions(+)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 339ee84e5a61..11641f924d08 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -350,9 +350,18 @@ void __init arm64_memblock_init(void)
>  			"initrd not fully accessible via the linear mapping -- please check your bootloader ...\n")) {
>  			phys_initrd_size = 0;
>  		} else {
> +			int flags, ret;
> +
> +			ret = memblock_get_flags(base, &flags);
> +			if (ret)
> +				flags = 0;
> +
>  			memblock_remove(base, size); /* clear MEMBLOCK_ flags */
>  			memblock_add(base, size);
>  			memblock_reserve(base, size);

Can you explain why we're removing+re-adding here exactly? Is it just to
clear flags as the comment indicates?


If it's really just about clearing flags, I wonder if we rather want to
have an interface that does exactly that, and hides the way this is
actually implemented (obtain flags, remove, re-add ...), internally.

But most probably there is more magic in the code and clearing flags
isn't all it ends up doing.

-- 
Thanks,

David / dhildenb