* [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache
@ 2014-07-02 3:42 Liu Ping Fan
2014-07-02 4:29 ` Hugh Dickins
0 siblings, 1 reply; 3+ messages in thread
From: Liu Ping Fan @ 2014-07-02 3:42 UTC (permalink / raw)
To: linux-mm; +Cc: Andrew Morton, Hugh Dickins
There is race between do_swap_page() and swap_writepage(), if
do_swap_page() had deleted a page from swap cache, there is no need
to write it. So changing the ret of try_to_free_swap() to make
swap_writepage() aware of this scene.
Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
mm/swapfile.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4c524f7..9d80671 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -910,7 +910,7 @@ int try_to_free_swap(struct page *page)
VM_BUG_ON_PAGE(!PageLocked(page), page);
if (!PageSwapCache(page))
- return 0;
+ return -1;
if (PageWriteback(page))
return 0;
if (page_swapcount(page))
--
1.8.1.4
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^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache
2014-07-02 3:42 [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache Liu Ping Fan
@ 2014-07-02 4:29 ` Hugh Dickins
2014-07-02 6:50 ` Liu ping fan
0 siblings, 1 reply; 3+ messages in thread
From: Hugh Dickins @ 2014-07-02 4:29 UTC (permalink / raw)
To: Liu Ping Fan; +Cc: linux-mm, Andrew Morton
On Wed, 2 Jul 2014, Liu Ping Fan wrote:
> There is race between do_swap_page() and swap_writepage(), if
> do_swap_page() had deleted a page from swap cache, there is no need
> to write it. So changing the ret of try_to_free_swap() to make
> swap_writepage() aware of this scene.
Is this an inefficiency that you have noticed in practice,
or something that you think you spotted by code inspection?
I don't see how it can happen: all the places I know of that call
swap_writepage() (including vmscan.c's mapping->a_ops->writepage)
have not dropped page lock since setting or checking PageSwapCache,
and page lock is supposed to protect against deletion from swap cache.
Has that changed? Please point out where.
>
> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> ---
> mm/swapfile.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4c524f7..9d80671 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -910,7 +910,7 @@ int try_to_free_swap(struct page *page)
> VM_BUG_ON_PAGE(!PageLocked(page), page);
>
> if (!PageSwapCache(page))
> - return 0;
> + return -1;
Previously it returned either 0 or 1, which is what __try_to_reclaim_swap()
says it returns; so better to stick to 0 or 1, unless you have good reason
to add a distinct value.
It's true that by the time __try_to_reclaim_swap() has got the page lock,
the page might have been removed from swap cache, and we could then treat
that as swap_was_freed (even though it was not freed by the caller).
But it's a very narrow window, and no great advantage to do so:
I don't think it's worth changing try_to_free_swap() semantics for,
but you could persuade us.
Hugh
> if (PageWriteback(page))
> return 0;
> if (page_swapcount(page))
> --
> 1.8.1.4
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache
2014-07-02 4:29 ` Hugh Dickins
@ 2014-07-02 6:50 ` Liu ping fan
0 siblings, 0 replies; 3+ messages in thread
From: Liu ping fan @ 2014-07-02 6:50 UTC (permalink / raw)
To: Hugh Dickins; +Cc: linux-mm, Andrew Morton
On Wed, Jul 2, 2014 at 12:29 PM, Hugh Dickins <hughd@google.com> wrote:
> On Wed, 2 Jul 2014, Liu Ping Fan wrote:
>
>> There is race between do_swap_page() and swap_writepage(), if
>> do_swap_page() had deleted a page from swap cache, there is no need
>> to write it. So changing the ret of try_to_free_swap() to make
>> swap_writepage() aware of this scene.
>
> Is this an inefficiency that you have noticed in practice,
> or something that you think you spotted by code inspection?
>
just spotted by code inspection.
> I don't see how it can happen: all the places I know of that call
> swap_writepage() (including vmscan.c's mapping->a_ops->writepage)
> have not dropped page lock since setting or checking PageSwapCache,
> and page lock is supposed to protect against deletion from swap cache.
>
> Has that changed? Please point out where.
>
No, my fault. Thanks for making me aware of this.
>>
>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>> ---
>> mm/swapfile.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 4c524f7..9d80671 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -910,7 +910,7 @@ int try_to_free_swap(struct page *page)
>> VM_BUG_ON_PAGE(!PageLocked(page), page);
>>
>> if (!PageSwapCache(page))
>> - return 0;
>> + return -1;
>
> Previously it returned either 0 or 1, which is what __try_to_reclaim_swap()
> says it returns; so better to stick to 0 or 1, unless you have good reason
> to add a distinct value.
>
> It's true that by the time __try_to_reclaim_swap() has got the page lock,
> the page might have been removed from swap cache, and we could then treat
> that as swap_was_freed (even though it was not freed by the caller).
>
> But it's a very narrow window, and no great advantage to do so:
> I don't think it's worth changing try_to_free_swap() semantics for,
> but you could persuade us.
>
Got it, and it is meaningless to do that.
Thanks,
Fan
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^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2014-07-02 6:50 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2014-07-02 3:42 [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache Liu Ping Fan
2014-07-02 4:29 ` Hugh Dickins
2014-07-02 6:50 ` Liu ping fan
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