Re: [PATCH] mm: swap: avoid to writepage when a page is !PageSwapCache

[Hugh Dickins <hughd@google.com>]

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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