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

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Date:	Sat, 16 Jun 2012 03:05:26 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
cc:	Wanpeng Li <liwp.linux@...il.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, 16 Jun 2012, Cong Wang wrote:
> 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))
> > 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)

Correct.

And that is not the limiting condition on available swap pages on 32-bit
without PAE, which is limited more by the pte<->swp conversion: a swap
entry must be distinguished from a present pte, from a PROT_NONE page,
and from a pte_file() entry - see arch/x86/include/asm/pgtable-2level.h
for how i386 in particular arranges that.

Nor is it the limiting condition on 64-bit, where include/linux/swap.h's
use of __u32 and unsigned int for counting swap pages is more limiting.

Hugh