Re: [v3 3/4] memblock: introduce MEMBLOCK_RSRV_NOINIT_VMEMMAP flag

[Muchun Song <muchun.song@linux.dev>]

On 2023/8/28 15:47, Mike Rapoport wrote:
> On Fri, Aug 25, 2023 at 12:18:35PM +0100, 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 | 10 ++++++++++
>>   mm/memblock.c            | 32 +++++++++++++++++++++++++++-----
>>   2 files changed, 37 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>> index f71ff9f0ec81..6d681d053880 100644
>> --- a/include/linux/memblock.h
>> +++ b/include/linux/memblock.h
>> @@ -40,6 +40,8 @@ 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_VMEMMAP: memory region for which struct pages are
>> + * not initialized (only for reserved regions).
>>    */
>>   enum memblock_flags {
>>   	MEMBLOCK_NONE		= 0x0,	/* No special request */
>> @@ -47,6 +49,8 @@ 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 */
>> +	/* don't initialize struct pages associated with this reserver memory block */
>> +	MEMBLOCK_RSRV_NOINIT_VMEMMAP	= 0x10,
> The flag means that struct page shouldn't be initialized, it may be used
> not only by vmemmap optimizations.
> Please drop _VMEMMAP.

The area at where the struct pages located is vmemmap, I think the
"vmemap" suffix does not mean that it is for "vmemmap optimization",
it could specify the target which will not be initialized. For me,
MEMBLOCK_RSRV_NOINIT does not tell me what should not be initialized,
memblock itself or its struct page (aka vmemmap pages)? So maybe
the suffix is better to keep?

>
> And I agree with Muchun's remarks about the comments.
>
>
>
>>   };
>>   
>>   /**
>> @@ -125,6 +129,7 @@ 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_reserved_mark_noinit_vmemmap(phys_addr_t base, phys_addr_t size);
> memblock does not care about vmemmap, please drop _vmemmap here and below as well.
>    
>>   void memblock_free_all(void);
>>   void memblock_free(void *ptr, size_t size);
>> @@ -259,6 +264,11 @@ static inline bool memblock_is_nomap(struct memblock_region *m)
>>   	return m->flags & MEMBLOCK_NOMAP;
>>   }
>>   
>> +static inline bool memblock_is_noinit_vmemmap(struct memblock_region *m)
> memblock_is_reserved_noinit please.
>
>> +{
>> +	return m->flags & MEMBLOCK_RSRV_NOINIT_VMEMMAP;
>> +}
>> +
>>   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 43cb4404d94c..a9782228c840 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -991,6 +991,23 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>>   	return memblock_setclr_flag(&memblock.memory, base, size, 0, MEMBLOCK_NOMAP);
>>   }
>>   
>> +/**
>> + * memblock_reserved_mark_noinit_vmemmap - Mark a reserved memory region with flag
>> + * MEMBLOCK_RSRV_NOINIT_VMEMMAP.
> this should be about what marking RSRV_NOINIT does, not what flag it uses
>
>> + * @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_VMEMMAP.
>> + *
>> + * Return: 0 on success, -errno on failure.
>> + */
>> +int __init_memblock memblock_reserved_mark_noinit_vmemmap(phys_addr_t base, phys_addr_t size)
>> +{
>> +	return memblock_setclr_flag(&memblock.reserved, base, size, 1,
>> +				    MEMBLOCK_RSRV_NOINIT_VMEMMAP);
>> +}
>> +
>>   static bool should_skip_region(struct memblock_type *type,
>>   			       struct memblock_region *m,
>>   			       int nid, int flags)
>> @@ -2107,13 +2124,18 @@ 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_VMEMMAP flag set
>> +	 */
>>   	for_each_reserved_mem_region(region) {
>> -		nid = memblock_get_region_node(region);
>> -		start = region->base;
>> -		end = start + region->size;
>> +		if (!memblock_is_noinit_vmemmap(region)) {
>> +			nid = memblock_get_region_node(region);
>> +			start = region->base;
>> +			end = start + region->size;
>>   
>> -		reserve_bootmem_region(start, end, nid);
>> +			reserve_bootmem_region(start, end, nid);
>> +		}
>>   	}
>>   }
>>   
>> -- 
>> 2.25.1
>>