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

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

On Thu, 1 Apr 2010 18:36:41 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> > > ==
> > > A  A  A  A skip_remap = 0;
> > > A  A  A  A if (PageAnon(page)) {
> > > A  A  A  A  A  A  A  A rcu_read_lock();
> > > A  A  A  A  A  A  A  A if (!page_mapped(page)) {
> > > A  A  A  A  A  A  A  A  A  A  A  A if (!PageSwapCache(page))
> > > A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A goto rcu_unlock;
> > > A  A  A  A  A  A  A  A  A  A  A  A /*
> > > A  A  A  A  A  A  A  A  A  A  A  A  * We can't convice this anon_vma is valid or not because
> > > A  A  A  A  A  A  A  A  A  A  A  A  * !page_mapped(page). Then, we do migration(radix-tree replacement)
> > > A  A  A  A  A  A  A  A  A  A  A  A  * but don't remap it which touches anon_vma in page->mapping.
> > > A  A  A  A  A  A  A  A  A  A  A  A  */
> > > A  A  A  A  A  A  A  A  A  A  A  A skip_remap = 1;
> > > A  A  A  A  A  A  A  A  A  A  A  A goto skip_unmap;
> > > A  A  A  A  A  A  A  A } else {
> > > A  A  A  A  A  A  A  A  A  A  A  A anon_vma = page_anon_vma(page);
> > > A  A  A  A  A  A  A  A  A  A  A  A atomic_inc(&anon_vma->external_refcount);
> > > A  A  A  A  A  A  A  A }
> > > A  A  A  A }
> > > A  A  A  A .....copy page, radix-tree replacement,....
> > >
> > 
> > It's not enough.
> > we uses remove_migration_ptes in  move_to_new_page, too.
> > We have to prevent it.
> > We can check PageSwapCache(page) in move_to_new_page and then
> > skip remove_migration_ptes.
> > 
> > ex)
> > static int move_to_new_page(....)
> > {
> >      int swapcache = PageSwapCache(page);
> >      ...
> >      if (!swapcache)
> >          if(!rc)
> >              remove_migration_ptes
> >          else
> >              newpage->mapping = NULL;
> > }
> > 
> 
> This I agree with.
> 
me, too.


> I am not sure this race exists because the page is locked but a key
> observation has been made - A page that is unmapped can be migrated if
> it's PageSwapCache but it may not have a valid anon_vma. Hence, in the
> !page_mapped case, the key is to not use anon_vma. How about the
> following patch?
> 

Seems good to me. But (see below)


> ==== CUT HERE ====
> 
> mm,migration: Allow the migration of PageSwapCache pages
> 
> 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 but ensures that no attempt to made to remap the pages
> would would potentially try to access an already freed anon_vma.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 35aad2a..5d0218b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -484,7 +484,8 @@ static int fallback_migrate_page(struct address_space *mapping,
>   *   < 0 - error code
>   *  == 0 - success
>   */
> -static int move_to_new_page(struct page *newpage, struct page *page)
> +static int move_to_new_page(struct page *newpage, struct page *page,
> +						int safe_to_remap)
>  {
>  	struct address_space *mapping;
>  	int rc;
> @@ -519,10 +520,12 @@ static int move_to_new_page(struct page *newpage, struct page *page)
>  	else
>  		rc = fallback_migrate_page(mapping, newpage, page);
>  
> -	if (!rc)
> -		remove_migration_ptes(page, newpage);
> -	else
> -		newpage->mapping = NULL;
> +	if (safe_to_remap) {
> +		if (!rc)
> +			remove_migration_ptes(page, newpage);
> +		else
> +			newpage->mapping = NULL;
> +	}
>  
	if (rc)
		newpage->mapping = NULL;
	else if (safe_to_remap)
		remove_migrateion_ptes(page, newpage);

Is better. Old code cleared newpage->mapping if rc!=0.

Thanks,
-Kame




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