linux-kernel - Re: [PATCH V2] mm/thp: Make ALLOC_SPLIT_PTLOCKS dependent on USE_SPLIT_PTE

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Message-ID: <9d1ce685-e0fd-febd-5ff2-179f7fa6e3fa@arm.com>
Date:   Thu, 1 Jul 2021 10:51:27 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/thp: Make ALLOC_SPLIT_PTLOCKS dependent on
 USE_SPLIT_PTE_PTLOCKS



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 */