Re: [PATCH -v3 01/10] mm, swap: Make swap cluster size same of THP size on x86_64

["Huang\, Ying" <ying.huang@intel.com>]

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Wed, Sep 07, 2016 at 09:46:00AM -0700, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>> 
>> In this patch, the size of the swap cluster is changed to that of the
>> THP (Transparent Huge Page) on x86_64 architecture (512).  This is for
>> the THP swap support on x86_64.  Where one swap cluster will be used to
>> hold the contents of each THP swapped out.  And some information of the
>> swapped out THP (such as compound map count) will be recorded in the
>> swap_cluster_info data structure.
>> 
>> For other architectures which want THP swap support, THP_SWAP_CLUSTER
>> need to be selected in the Kconfig file for the architecture.
>> 
>> In effect, this will enlarge swap cluster size by 2 times on x86_64.
>> Which may make it harder to find a free cluster when the swap space
>> becomes fragmented.  So that, this may reduce the continuous swap space
>> allocation and sequential write in theory.  The performance test in 0day
>> shows no regressions caused by this.
>> 
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Shaohua Li <shli@kernel.org>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Rik van Riel <riel@redhat.com>
>> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> ---
>>  arch/x86/Kconfig |  1 +
>>  mm/Kconfig       | 13 +++++++++++++
>>  mm/swapfile.c    |  4 ++++
>>  3 files changed, 18 insertions(+)
>> 
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 4c39728..421d862 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -164,6 +164,7 @@ config X86
>>  	select HAVE_STACK_VALIDATION		if X86_64
>>  	select ARCH_USES_HIGH_VMA_FLAGS		if X86_INTEL_MEMORY_PROTECTION_KEYS
>>  	select ARCH_HAS_PKEYS			if X86_INTEL_MEMORY_PROTECTION_KEYS
>> +	select ARCH_USES_THP_SWAP_CLUSTER	if X86_64
>>  
>>  config INSTRUCTION_DECODER
>>  	def_bool y
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index be0ee11..2da8128 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -503,6 +503,19 @@ config FRONTSWAP
>>  
>>  	  If unsure, say Y to enable frontswap.
>>  
>> +config ARCH_USES_THP_SWAP_CLUSTER
>> +	bool
>> +	default n
>> +
>> +config THP_SWAP_CLUSTER
>> +	bool
>> +	depends on SWAP && TRANSPARENT_HUGEPAGE && ARCH_USES_THP_SWAP_CLUSTER
>> +	default y
>> +	help
>> +	  Use one swap cluster to hold the contents of the THP
>> +	  (Transparent Huge Page) swapped out.  The size of the swap
>> +	  cluster will be same as that of THP.
>> +
>
> Why do we need to ask user about it? I don't think most users qualified to
> make this decision.

Users need not to choose this.  If the dependencies is true, it will be
turned on.  I added the help here not for users, but for developers to
know what it is for.

>>  config CMA
>>  	bool "Contiguous Memory Allocator"
>>  	depends on HAVE_MEMBLOCK && MMU
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 8f1b97d..4b78402 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -196,7 +196,11 @@ static void discard_swap_cluster(struct swap_info_struct *si,
>>  	}
>>  }
>>  
>> +#ifdef CONFIG_THP_SWAP_CLUSTER
>
> Just
>
> #if defined(CONFIG_ARCH_USES_THP_SWAP_CLUSTER) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
>
> would be enough from my POV.

That works.  I added a new configuration option just to save some typing
and make it a little easier to read.  If other people think it is not
necessary to add a new configuration option for that too.  I will use
change it in this way.

Best Regards,
Huang, Ying

>> +#define SWAPFILE_CLUSTER	(HPAGE_SIZE / PAGE_SIZE)
>> +#else
>>  #define SWAPFILE_CLUSTER	256
>> +#endif
>>  #define LATENCY_LIMIT		256
>>  
>>  static inline void cluster_set_flag(struct swap_cluster_info *info,
>> -- 
>> 2.8.1
>>