linux-kernel - Re: [PATCH v3] mm, thp: introduce generic transparent huge page allocation interfaces

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Message-Id: <20171207154519.df3f8218f8dbe05f95a2bc42@linux-foundation.org>
Date:   Thu, 7 Dec 2017 15:45:19 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     changbin.du@...el.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm, thp: introduce generic transparent huge page
 allocation interfaces

On Thu,  7 Dec 2017 18:54:19 +0800 changbin.du@...el.com wrote:

> From: Changbin Du <changbin.du@...el.com>
> 
> This patch introduced 4 new interfaces to allocate a prepared transparent
> huge page. These interfaces merge distributed two-step allocation as simple
> single step. And they can avoid issue like forget to call prep_transhuge_page()
> or call it on wrong page. A real fix:
> 40a899e ("mm: migrate: fix an incorrect call of prep_transhuge_page()")
> 
> Anyway, I just want to prove that expose direct allocation interfaces is
> better than a interface only do the second part of it.
> 
> These are similar to alloc_hugepage_xxx which are for hugetlbfs pages. New
> interfaces are:
>   - alloc_transhuge_page_vma
>   - alloc_transhuge_page_nodemask
>   - alloc_transhuge_page_node
>   - alloc_transhuge_page
> 
> These interfaces implicitly add __GFP_COMP gfp mask which is the minimum
> flags used for huge page allocation. More flags leave to the callers.
> 
> This patch does below changes:
>   - define alloc_transhuge_page_xxx interfaces
>   - apply them to all existing code
>   - declare prep_transhuge_page as static since no others use it
>   - remove alloc_hugepage_vma definition since it no longer has users
> 
> ...
>
> @@ -261,7 +272,10 @@ static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>  	return false;
>  }
>  
> -static inline void prep_transhuge_page(struct page *page) {}
> +#define alloc_transhuge_page_vma(gfp_mask, vma, addr) NULL
> +#define alloc_transhuge_page_nodemask(gfp_mask, preferred_nid, nmask) NULL
> +#define alloc_transhuge_page_node(nid, gfp_maskg) NULL
> +#define alloc_transhuge_page(gfp_mask) NULL

Ugly.  And such things can cause unused-variable warnings in calling
code.  Whereas

static inline struct page *alloc_transhuge_page_vma(gfp_t gfp_mask,
			struct vm_area_struct *vma, unsigned long addr)
{
	return NULL;
}

will avoid such warnings.