linux-kernel - Re: [PATCH] swap: fix shmem swapping when more than 8 areas

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Message-ID: <CAM_iQpXPH2SgjKbj1g5azcddusBmQ0CDvDz_RJe2r2HSTo51yA@mail.gmail.com>
Date:	Sat, 16 Jun 2012 14:10:56 +0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Wanpeng Li <liwp.linux@...il.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] swap: fix shmem swapping when more than 8 areas

On Sat, Jun 16, 2012 at 12:56 PM, Wanpeng Li <liwp.linux@...il.com> wrote:
>>-#define SWP_TYPE_SHIFT(e)     (sizeof(e.val) * 8 - MAX_SWAPFILES_SHIFT)
>>+#define SWP_TYPE_SHIFT(e)     ((sizeof(e.val) * 8) - \
>>+                      (MAX_SWAPFILES_SHIFT + RADIX_TREE_EXCEPTIONAL_SHIFT))
>
> Hi Hugh,
>
> Since SHIFT == MAX_SWAPFILES_SHIFT + RADIX_TREE_EXCEPTIONAL_SHIFT == 7
> and the low two bits used for radix_tree, the available swappages number
> based of 32bit architectures reduce to 2^(32-7-2) = 32GB?
>

The lower two bits are in the 7 bits you calculated,
so it is 2^(32-7), not 2^(32-7-2)
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