Re: [PATCH 14/14] mm,migration: Allow the migration of PageSwapCache pages

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 30 Mar 2010 10:14:49 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> PageAnon pages that are unmapped may or may not have an anon_vma so
> are not currently migrated. However, a swap cache page can be migrated
> and fits this description. This patch identifies page swap caches and
> allows them to be migrated.
> 

Some comments.

> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  mm/migrate.c |   15 ++++++++++-----
>  mm/rmap.c    |    6 ++++--
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 35aad2a..f9bf37e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -203,6 +203,9 @@ static int migrate_page_move_mapping(struct address_space *mapping,
>  	void **pslot;
>  
>  	if (!mapping) {
> +		if (PageSwapCache(page))
> +			SetPageSwapCache(newpage);
> +

Migration of SwapCache requires radix-tree replacement, IOW, 
 mapping == NULL && PageSwapCache is BUG.

So, this never happens.


>  		/* Anonymous page without mapping */
>  		if (page_count(page) != 1)
>  			return -EAGAIN;
> @@ -607,11 +610,13 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		 * the page was isolated and when we reached here while
>  		 * the RCU lock was not held
>  		 */
> -		if (!page_mapped(page))
> -			goto rcu_unlock;
> -
> -		anon_vma = page_anon_vma(page);
> -		atomic_inc(&anon_vma->external_refcount);
> +		if (!page_mapped(page)) {
> +			if (!PageSwapCache(page))
> +				goto rcu_unlock;
> +		} else {
> +			anon_vma = page_anon_vma(page);
> +			atomic_inc(&anon_vma->external_refcount);
> +		}
>  	}
>  
>  	/*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index af35b75..d5ea1f2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1394,9 +1394,11 @@ int rmap_walk(struct page *page, int (*rmap_one)(struct page *,
>  
>  	if (unlikely(PageKsm(page)))
>  		return rmap_walk_ksm(page, rmap_one, arg);
> -	else if (PageAnon(page))
> +	else if (PageAnon(page)) {
> +		if (PageSwapCache(page))
> +			return SWAP_AGAIN;
>  		return rmap_walk_anon(page, rmap_one, arg);

SwapCache has a condition as (PageSwapCache(page) && page_mapped(page) == true.

Please see do_swap_page(), PageSwapCache bit is cleared only when

do_swap_page()...
       swap_free(entry);
        if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
                try_to_free_swap(page);

Then, PageSwapCache is cleared only when swap is freeable even if mapped.

rmap_walk_anon() should be called and the check is not necessary.

Thanks,
-Kame

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