Re: [PATCH V2] mm/thp: Make ALLOC_SPLIT_PTLOCKS dependent on USE_SPLIT_PTE_PTLOCKS

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/20/21 4:47 PM, Matthew Wilcox wrote:
> On Wed, May 19, 2021 at 01:03:06PM +0530, Anshuman Khandual wrote:
>> Split ptlocks need not be defined and allocated unless they are being used.
>> ALLOC_SPLIT_PTLOCKS is inherently dependent on USE_SPLIT_PTE_PTLOCKS. This
>> just makes it explicit and clear. While here drop the spinlock_t element
>> from the struct page when USE_SPLIT_PTE_PTLOCKS is not enabled.
> 
> I didn't spot this email yesterday.  I'm not a fan.  Isn't struct page
> already complicated enough without adding another ifdef to it?  Surely
> there's a better way than this.

This discussion thread just got dropped off the radar, sorry about it.
None of the spinlock_t elements are required unless split ptlocks are
in use. I understand your concern regarding yet another #ifdef in the
struct page definition. But this change is simple and minimal. Do you
have any other particular alternative in mind which I could explore ?

> 
>> +++ b/include/linux/mm_types.h
>> @@ -152,10 +152,12 @@ struct page {
>>  				struct mm_struct *pt_mm; /* x86 pgds only */
>>  				atomic_t pt_frag_refcount; /* powerpc */
>>  			};
>> +#if USE_SPLIT_PTE_PTLOCKS
>>  #if ALLOC_SPLIT_PTLOCKS
>>  			spinlock_t *ptl;
>>  #else
>>  			spinlock_t ptl;
>> +#endif
>>  #endif
>>  		};
>>  		struct {	/* ZONE_DEVICE pages */