linux-kernel - Re: [patch] hugetlb: correct page offset index for sharing pmd

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Message-ID: <20120803133235.GA8434@dhcp22.suse.cz>
Date:	Fri, 3 Aug 2012 15:32:35 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] hugetlb: correct page offset index for sharing pmd

On Fri 03-08-12 20:56:45, Hillf Danton wrote:
> The computation of page offset index is open coded, and incorrect, to
> be used in scanning prio tree, as huge page offset is required, and is
> fixed with the well defined routine.

I guess that nobody reported this because if someone really wants to
share he will use aligned address for mmap/shmat and so the index is 0.
Anyway it is worth fixing. Thanks for pointing out!

> 
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
> 
> --- a/arch/x86/mm/hugetlbpage.c	Fri Aug  3 20:34:58 2012
> +++ b/arch/x86/mm/hugetlbpage.c	Fri Aug  3 20:40:16 2012
> @@ -72,12 +72,15 @@ static void huge_pmd_share(struct mm_str
>  	if (!vma_shareable(vma, addr))
>  		return;
> 
> +	idx = linear_page_index(vma, addr);
> +

You can use linear_hugepage_index directly and remove the idx
initialization as well.

>  	mutex_lock(&mapping->i_mmap_mutex);
>  	vma_prio_tree_foreach(svma, &iter, &mapping->i_mmap, idx, idx) {
>  		if (svma == vma)
>  			continue;
> 
> -		saddr = page_table_shareable(svma, vma, addr, idx);
> +		saddr = page_table_shareable(svma, vma, addr,
> +						idx * (PMD_SIZE/PAGE_SIZE));

Why not just fixing page_table_shareable as well rather than playing
tricks like this?

>  		if (saddr) {
>  			spte = huge_pte_offset(svma->vm_mm, saddr);
>  			if (spte) {
> --

-- 
Michal Hocko
SUSE Labs
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