Re: [PATCH v2 3/5] mm: memory_hotplug: memblock to track partially removed vmemmap mem

[Michal Hocko <mhocko@kernel.org>]

On Thu 23-11-17 11:14:38, Andrea Reale wrote:
> When hot-removing memory we need to free vmemmap memory.
> However, depending on the memory is being removed, it might
> not be always possible to free a full vmemmap page / huge-page
> because part of it might still be used.
> 
> Commit ae9aae9eda2d ("memory-hotplug: common APIs to support page tables
> hot-remove") introduced a workaround for x86
> hot-remove, by which partially unused areas are filled with
> the 0xFD constant. Full pages are only removed when fully
> filled by 0xFDs.
> 
> This commit introduces a MEMBLOCK_UNUSED_VMEMMAP memblock flag, with
> the goal of using it in place of 0xFDs. For now, this will be used for
> the arm64 port of memory hot remove, but the idea is to eventually use
> the same mechanism for x86 as well.

Why cannot you use the same approach as x86 have? Have a look at the
vmemmap_free at al.
 
> Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
> Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
> ---
>  include/linux/memblock.h | 12 ++++++++++++
>  mm/memblock.c            | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index bae11c7..0daec05 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -26,6 +26,9 @@ enum {
>  	MEMBLOCK_HOTPLUG	= 0x1,	/* hotpluggable region */
>  	MEMBLOCK_MIRROR		= 0x2,	/* mirrored region */
>  	MEMBLOCK_NOMAP		= 0x4,	/* don't add to kernel direct mapping */
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +	MEMBLOCK_UNUSED_VMEMMAP	= 0x8,  /* Mark VMEMAP blocks as dirty */
> +#endif
>  };
>  
>  struct memblock_region {
> @@ -90,6 +93,10 @@ 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);
>  ulong choose_memblock_flags(void);
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +int memblock_mark_unused_vmemmap(phys_addr_t base, phys_addr_t size);
> +int memblock_clear_unused_vmemmap(phys_addr_t base, phys_addr_t size);
> +#endif
>  
>  /* Low level functions */
>  int memblock_add_range(struct memblock_type *type,
> @@ -182,6 +189,11 @@ static inline bool memblock_is_nomap(struct memblock_region *m)
>  	return m->flags & MEMBLOCK_NOMAP;
>  }
>  
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +bool memblock_is_vmemmap_unused_range(struct memblock_type *mt,
> +		phys_addr_t start, phys_addr_t end);
> +#endif
> +
>  #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
>  int memblock_search_pfn_nid(unsigned long pfn, unsigned long *start_pfn,
>  			    unsigned long  *end_pfn);
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 9120578..30d5aa4 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -809,6 +809,18 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>  	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
>  }
>  
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +int __init_memblock memblock_mark_unused_vmemmap(phys_addr_t base,
> +		phys_addr_t size)
> +{
> +	return memblock_setclr_flag(base, size, 1, MEMBLOCK_UNUSED_VMEMMAP);
> +}
> +int __init_memblock memblock_clear_unused_vmemmap(phys_addr_t base,
> +		phys_addr_t size)
> +{
> +	return memblock_setclr_flag(base, size, 0, MEMBLOCK_UNUSED_VMEMMAP);
> +}
> +#endif
>  /**
>   * __next_reserved_mem_region - next function for for_each_reserved_region()
>   * @idx: pointer to u64 loop variable
> @@ -1696,6 +1708,26 @@ void __init_memblock memblock_trim_memory(phys_addr_t align)
>  	}
>  }
>  
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +bool __init_memblock memblock_is_vmemmap_unused_range(struct memblock_type *mt,
> +		phys_addr_t start, phys_addr_t end)
> +{
> +	u64 i;
> +	struct memblock_region *r;
> +
> +	i = memblock_search(mt, start);
> +	r = &(mt->regions[i]);
> +	while (r->base < end) {
> +		if (!(r->flags & MEMBLOCK_UNUSED_VMEMMAP))
> +			return 0;
> +
> +		r = &(memblock.memory.regions[++i]);
> +	}
> +
> +	return 1;
> +}
> +#endif
> +
>  void __init_memblock memblock_set_current_limit(phys_addr_t limit)
>  {
>  	memblock.current_limit = limit;
> -- 
> 2.7.4
> 
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-- 
Michal Hocko
SUSE Labs