Re: [PATCH] mm/memblock: remove empty dummy entry

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 03, 2024 at 09:10:45AM +0000, Wei Yang wrote:
> The dummy entry is introduced in the initial implementation of lmb in
> commit 7c8c6b9776fb ("powerpc: Merge lmb.c and make MM initialization
> use it.").
> 
> As the comment says the empty dummy entry is to simplify the code.
> 
> 	/* Create a dummy zero size LMB which will get coalesced away later.
>          * This simplifies the lmb_add() code below...
>          */
> 
> While current code is reimplemented by Tejun in commit 784656f9c680
> ("memblock: Reimplement memblock_add_region()"). This empty dummy entry
> seems not benefit the code any more.
> 
> Let's remove it.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Paul Mackerras <paulus@ozlabs.org>
> CC: Tejun Heo <tj@kernel.org>
> CC: Mike Rapoport <rppt@kernel.org>
> ---
>  mm/memblock.c                            | 18 +++---------------
>  tools/testing/memblock/tests/basic_api.c |  8 ++++----
>  tools/testing/memblock/tests/common.c    |  4 ++--
>  3 files changed, 9 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index d09136e040d3..100d3b7ab499 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -114,12 +114,12 @@ static struct memblock_region memblock_physmem_init_regions[INIT_PHYSMEM_REGIONS
>  
>  struct memblock memblock __initdata_memblock = {
>  	.memory.regions		= memblock_memory_init_regions,
> -	.memory.cnt		= 1,	/* empty dummy entry */
> +	.memory.cnt		= 0,

No need to set memory.cnt to 0, this line can be just removed

>  	.memory.max		= INIT_MEMBLOCK_MEMORY_REGIONS,
>  	.memory.name		= "memory",
>  
>  	.reserved.regions	= memblock_reserved_init_regions,
> -	.reserved.cnt		= 1,	/* empty dummy entry */
> +	.reserved.cnt		= 0,

Ditto.

>  	.reserved.max		= INIT_MEMBLOCK_RESERVED_REGIONS,
>  	.reserved.name		= "reserved",
>  
> @@ -130,7 +130,7 @@ struct memblock memblock __initdata_memblock = {
>  #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
>  struct memblock_type physmem = {
>  	.regions		= memblock_physmem_init_regions,
> -	.cnt			= 1,	/* empty dummy entry */
> +	.cnt			= 0,

Ditto.

>  	.max			= INIT_PHYSMEM_REGIONS,
>  	.name			= "physmem",
>  };
> @@ -356,7 +356,6 @@ static void __init_memblock memblock_remove_region(struct memblock_type *type, u
>  	/* Special case for empty arrays */
>  	if (type->cnt == 0) {
>  		WARN_ON(type->total_size != 0);
> -		type->cnt = 1;
>  		type->regions[0].base = 0;
>  		type->regions[0].size = 0;
>  		type->regions[0].flags = 0;
> @@ -598,17 +597,6 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>  	if (!size)
>  		return 0;
>  
> -	/* special case for empty array */
> -	if (type->regions[0].size == 0) {
> -		WARN_ON(type->cnt != 1 || type->total_size);
> -		type->regions[0].base = base;
> -		type->regions[0].size = size;
> -		type->regions[0].flags = flags;
> -		memblock_set_region_node(&type->regions[0], nid);
> -		type->total_size = size;
> -		return 0;
> -	}

I'd keep the special case for empty array, just update it for new semantics
of type->cnt.

> -
>  	/*
>  	 * The worst case is when new range overlaps all existing regions,
>  	 * then we'll need type->cnt + 1 empty regions in @type. So if
> diff --git a/tools/testing/memblock/tests/basic_api.c b/tools/testing/memblock/tests/basic_api.c
> index 57bf2688edfd..f317fe691fc4 100644
> --- a/tools/testing/memblock/tests/basic_api.c
> +++ b/tools/testing/memblock/tests/basic_api.c
> @@ -15,12 +15,12 @@ static int memblock_initialization_check(void)
>  	PREFIX_PUSH();
>  
>  	ASSERT_NE(memblock.memory.regions, NULL);
> -	ASSERT_EQ(memblock.memory.cnt, 1);
> +	ASSERT_EQ(memblock.memory.cnt, 0);
>  	ASSERT_EQ(memblock.memory.max, EXPECTED_MEMBLOCK_REGIONS);
>  	ASSERT_EQ(strcmp(memblock.memory.name, "memory"), 0);
>  
>  	ASSERT_NE(memblock.reserved.regions, NULL);
> -	ASSERT_EQ(memblock.reserved.cnt, 1);
> +	ASSERT_EQ(memblock.reserved.cnt, 0);
>  	ASSERT_EQ(memblock.memory.max, EXPECTED_MEMBLOCK_REGIONS);
>  	ASSERT_EQ(strcmp(memblock.reserved.name, "reserved"), 0);
>  
> @@ -1295,7 +1295,7 @@ static int memblock_remove_only_region_check(void)
>  	ASSERT_EQ(rgn->base, 0);
>  	ASSERT_EQ(rgn->size, 0);
>  
> -	ASSERT_EQ(memblock.memory.cnt, 1);
> +	ASSERT_EQ(memblock.memory.cnt, 0);
>  	ASSERT_EQ(memblock.memory.total_size, 0);
>  
>  	test_pass_pop();
> @@ -1723,7 +1723,7 @@ static int memblock_free_only_region_check(void)
>  	ASSERT_EQ(rgn->base, 0);
>  	ASSERT_EQ(rgn->size, 0);
>  
> -	ASSERT_EQ(memblock.reserved.cnt, 1);
> +	ASSERT_EQ(memblock.reserved.cnt, 0);
>  	ASSERT_EQ(memblock.reserved.total_size, 0);
>  
>  	test_pass_pop();
> diff --git a/tools/testing/memblock/tests/common.c b/tools/testing/memblock/tests/common.c
> index f43b6f414983..4fa94b281280 100644
> --- a/tools/testing/memblock/tests/common.c
> +++ b/tools/testing/memblock/tests/common.c
> @@ -40,13 +40,13 @@ void reset_memblock_regions(void)
>  {
>  	memset(memblock.memory.regions, 0,
>  	       memblock.memory.cnt * sizeof(struct memblock_region));
> -	memblock.memory.cnt	= 1;
> +	memblock.memory.cnt	= 0;

Here as well, no need to initialize to 0.

>  	memblock.memory.max	= INIT_MEMBLOCK_REGIONS;
>  	memblock.memory.total_size = 0;
>  
>  	memset(memblock.reserved.regions, 0,
>  	       memblock.reserved.cnt * sizeof(struct memblock_region));
> -	memblock.reserved.cnt	= 1;
> +	memblock.reserved.cnt	= 0;
>  	memblock.reserved.max	= INIT_MEMBLOCK_RESERVED_REGIONS;
>  	memblock.reserved.total_size = 0;
>  }
> -- 
> 2.34.1
> 

-- 
Sincerely yours,
Mike.