On Tue, 30 Mar 2010 10:14:49 +0100
Mel Gorman <mel@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> 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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