[PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Hua Zhong]

Hi,

can_share_swap_page() is used to check if the page has the last reference. This avoids allocating a new page for COW if it's the last page.

However, if CONFIG_SWAP is not set, can_share_swap_page() is defined as 0, thus always causes a copy for the last COW page. The below simple patch fixes it.

I'm not sure if it's the best fix. Maybe we should rename can_share_swap_page() and move it out of swapfile.c. Comments?

Signed-off-by: Hua Zhong <hzhong@gmail.com>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 5b1fdf1..f03c247 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -296,7 +296,7 @@ #define swap_free(swp)				/*NOTHING*/
 #define read_swap_cache_async(swp,vma,addr)	NULL
 #define lookup_swap_cache(swp)			NULL
 #define valid_swaphandles(swp, off)		0
-#define can_share_swap_page(p)			0
+#define can_share_swap_page(p)			(page_mapcount(p) == 1)
 #define move_to_swap_cache(p, swp)		1
 #define move_from_swap_cache(p, i, m)		1
 #define __delete_from_swap_cache(p)		/*NOTHING*/

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Re: [PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Nick Piggin]

Hua Zhong wrote:

>Hi,
>
>can_share_swap_page() is used to check if the page has the last reference. This avoids allocating a new page for COW if it's the last page.
>
>However, if CONFIG_SWAP is not set, can_share_swap_page() is defined as 0, thus always causes a copy for the last COW page. The below simple patch fixes it.
>
>I'm not sure if it's the best fix. Maybe we should rename can_share_swap_page() and move it out of swapfile.c. Comments?
>

Looks like a good patch, nice catch. You should run it past Hugh but tend to
agree it would be nice to reuse the out of line can_share_swap_page, 
which would
fold beautifully with PageSwapCache a constant 0.

Nick
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Re: [PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Hugh Dickins]

On Mon, 8 May 2006, Nick Piggin wrote:
> Hua Zhong wrote:
> >
> >I'm not sure if it's the best fix. Maybe we should rename
> >can_share_swap_page() and move it out of swapfile.c. Comments?
> 
> Looks like a good patch, nice catch. You should run it past Hugh but tend to
> agree it would be nice to reuse the out of line can_share_swap_page, which
> would fold beautifully with PageSwapCache a constant 0.

True; but I think Hua's patch is good as is for now, to fix
that inefficiency.  I do agree (as you know) that there's scope for
cleanup there, and that that function is badly named; but I'm still
unprepared to embark on the cleanup, so let's just get the fix in.

Hugh

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Re: [PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Nick Piggin]

Hugh Dickins wrote:

>On Mon, 8 May 2006, Nick Piggin wrote:
>
>>Looks like a good patch, nice catch. You should run it past Hugh but tend to
>>agree it would be nice to reuse the out of line can_share_swap_page, which
>>would fold beautifully with PageSwapCache a constant 0.
>>
>
>True; but I think Hua's patch is good as is for now, to fix
>that inefficiency.  I do agree (as you know) that there's scope for
>cleanup there, and that that function is badly named; but I'm still
>unprepared to embark on the cleanup, so let's just get the fix in.
>

Sure. Queue it up for 2.6.18?
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Re: [PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Hugh Dickins]

On Tue, 9 May 2006, Nick Piggin wrote:
> Hugh Dickins wrote:
> >
> >True; but I think Hua's patch is good as is for now, to fix
> >that inefficiency.  I do agree (as you know) that there's scope for
> >cleanup there, and that that function is badly named; but I'm still
> >unprepared to embark on the cleanup, so let's just get the fix in.
> 
> Sure. Queue it up for 2.6.18?

I'd be perfectly happy for Hua's one-liner to go into 2.6.17;
but that's up to Andrew.

Hugh

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