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Date:   Fri, 28 Jul 2023 07:30:29 +0300
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Subject: Re: [v1 4/6] memblock: introduce MEMBLOCK_RSRV_NOINIT flag
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To:     Usama Arif <usama.arif@bytedance.com>, linux-mm@kvack.org,
        muchun.song@linux.dev, mike.kravetz@oracle.com, rppt@kernel.org
Cc:     linux-kernel@vger.kernel.org, fam.zheng@bytedance.com,
        liangma@liangbit.com, simon.evans@bytedance.com,
        punit.agrawal@bytedance.com
References: <20230727204624.1942372-1-usama.arif@bytedance.com>
 <20230727204624.1942372-5-usama.arif@bytedance.com>
From:   =?UTF-8?Q?Mika_Penttil=c3=a4?= <mpenttil@redhat.com>
In-Reply-To: <20230727204624.1942372-5-usama.arif@bytedance.com>
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Hi,

On 7/27/23 23:46, Usama Arif wrote:

> For reserved memory regions marked with this flag,
> reserve_bootmem_region is not called during memmap_init_reserved_pages.
> This can be used to avoid struct page initialization for
> regions which won't need them, for e.g. hugepages with
> HVO enabled.
>
> Signed-off-by: Usama Arif <usama.arif@bytedance.com>
> ---
>   include/linux/memblock.h |  7 +++++++
>   mm/memblock.c            | 32 ++++++++++++++++++++++++++------
>   2 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index f71ff9f0ec81..7f9d06c08592 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -47,6 +47,7 @@ enum memblock_flags {
>   	MEMBLOCK_MIRROR		= 0x2,	/* mirrored region */
>   	MEMBLOCK_NOMAP		= 0x4,	/* don't add to kernel direct mapping */
>   	MEMBLOCK_DRIVER_MANAGED = 0x8,	/* always detected via a driver */
> +	MEMBLOCK_RSRV_NOINIT	= 0x10,	/* don't call reserve_bootmem_region for this region */
>   };
>   
>   /**
> @@ -125,6 +126,7 @@ int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
>   int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
>   int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
>   int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
> +int memblock_rsrv_mark_noinit(phys_addr_t base, phys_addr_t size);
>   
>   void memblock_free_all(void);
>   void memblock_free(void *ptr, size_t size);
> @@ -259,6 +261,11 @@ static inline bool memblock_is_nomap(struct memblock_region *m)
>   	return m->flags & MEMBLOCK_NOMAP;
>   }
>   
> +static inline bool memblock_is_noinit(struct memblock_region *m)
> +{
> +	return m->flags & MEMBLOCK_RSRV_NOINIT;
> +}
> +
>   static inline bool memblock_is_driver_managed(struct memblock_region *m)
>   {
>   	return m->flags & MEMBLOCK_DRIVER_MANAGED;
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 4fd431d16ef2..3a15708af3b6 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -997,6 +997,22 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>   	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP, 0);
>   }
>   
> +/**
> + * memblock_rsrv_mark_noinit - Mark a reserved memory region with flag MEMBLOCK_RSRV_NOINIT.
> + * @base: the base phys addr of the region
> + * @size: the size of the region
> + *
> + * For memory regions marked with %MEMBLOCK_RSRV_NOINIT, reserve_bootmem_region
> + * is not called during memmap_init_reserved_pages, hence struct pages are not
> + * initialized for this region.
> + *
> + * Return: 0 on success, -errno on failure.
> + */
> +int __init_memblock memblock_rsrv_mark_noinit(phys_addr_t base, phys_addr_t size)
> +{
> +	return memblock_setclr_flag(base, size, 1, MEMBLOCK_RSRV_NOINIT, 1);
> +}
> +
>   static bool should_skip_region(struct memblock_type *type,
>   			       struct memblock_region *m,
>   			       int nid, int flags)
> @@ -2113,13 +2129,17 @@ static void __init memmap_init_reserved_pages(void)
>   		memblock_set_node(start, end, &memblock.reserved, nid);
>   	}
>   
> -	/* initialize struct pages for the reserved regions */
> +	/*
> +	 * initialize struct pages for reserved regions that don't have
> +	 * the MEMBLOCK_RSRV_NOINIT flag set
> +	 */
>   	for_each_reserved_mem_region(region) {
> -		nid = memblock_get_region_node(region);
> -		start = region->base;
> -		end = start + region->size;
> -
> -		reserve_bootmem_region(start, end, nid);
> +		if (!memblock_is_noinit(region)) {
> +			nid = memblock_get_region_node(region);
> +			start = region->base;
> +			end = start + region->size;
> +			reserve_bootmem_region(start, end, nid);
> +		}
>   	}
>   }
>   

There's code like:

static inline void free_vmemmap_page(struct page *page)
{
         if (PageReserved(page))
                 free_bootmem_page(page);
         else
                 __free_page(page);
}

which depends on the PageReserved being in vmempages pages, so I think you can't skip that part?

--Mika