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SubjectRe: [PATCH 14/14] mm,migration: Allow the migration of PageSwapCache pages
On Thu, 1 Apr 2010 18:36:41 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> > > ==
> > >        skip_remap = 0;
> > >        if (PageAnon(page)) {
> > >                rcu_read_lock();
> > >                if (!page_mapped(page)) {
> > >                        if (!PageSwapCache(page))
> > >                                goto rcu_unlock;
> > >                        /*
> > >                         * We can't convice this anon_vma is valid or not because
> > >                         * !page_mapped(page). Then, we do migration(radix-tree replacement)
> > >                         * but don't remap it which touches anon_vma in page->mapping.
> > >                         */
> > >                        skip_remap = 1;
> > >                        goto skip_unmap;
> > >                } else {
> > >                        anon_vma = page_anon_vma(page);
> > >                        atomic_inc(&anon_vma->external_refcount);
> > >                }
> > >        }
> > >        .....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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