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

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Date:   Thu, 20 May 2021 12:17:24 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
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 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.

> +++ 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 */