linux-kernel - Re: [PATCHv4 09/39] thp, mm: introduce mapping_can_have

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Date:	Tue, 21 May 2013 12:28:13 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 09/39] thp, mm: introduce mapping_can_have_hugepages()
 predicate

On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> Returns true if mapping can have huge pages. Just check for __GFP_COMP
> in gfp mask of the mapping for now.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  include/linux/pagemap.h |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index e3dea75..28597ec 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -84,6 +84,18 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
>  				(__force unsigned long)mask;
>  }
>  
> +static inline bool mapping_can_have_hugepages(struct address_space *m)
> +{
> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE)) {
> +		gfp_t gfp_mask = mapping_gfp_mask(m);
> +		/* __GFP_COMP is key part of GFP_TRANSHUGE */
> +		return !!(gfp_mask & __GFP_COMP) &&
> +			transparent_hugepage_pagecache();
> +	}
> +
> +	return false;
> +}

transparent_hugepage_pagecache() already has the same IS_ENABLED()
check,  Is it really necessary to do it again here?

IOW, can you do this?

> +static inline bool mapping_can_have_hugepages(struct address_space
> +{
> +		gfp_t gfp_mask = mapping_gfp_mask(m);
		if (!transparent_hugepage_pagecache())
			return false;
> +		/* __GFP_COMP is key part of GFP_TRANSHUGE */
> +		return !!(gfp_mask & __GFP_COMP);
> +}

I know we talked about this in the past, but I've forgotten already.
Why is this checking for __GFP_COMP instead of GFP_TRANSHUGE?

Please flesh out the comment.

Also, what happens if "transparent_hugepage_flags &
(1<<TRANSPARENT_HUGEPAGE_PAGECACHE)" becomes false at runtime and you
have some already-instantiated huge page cache mappings around?  Will
things like mapping_align_mask() break?
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