On Wed, Jan 05, 2011 at 01:00:20PM +0900, Daisuke Nishimura wrote:
> In current implimentation, mem_cgroup_end_migration() decides whether the page
> migration has succeeded or not by checking "oldpage->mapping".
>
> But if we are tring to migrate a shmem swapcache, the page->mapping of it is
> NULL from the begining, so the check would be invalid.
> As a result, mem_cgroup_end_migration() assumes the migration has succeeded
> even if it's not, so "newpage" would be freed while it's not uncharged.
>
> This patch fixes it by passing mem_cgroup_end_migration() the result of the
> page migration.
Are there other users that rely on unused->mapping being NULL after
migration?
If so, aren't they prone to misinterpreting this for shmem swapcache
as well?
If not, wouldn't it be better to remove that page->mapping = NULL from
migrate_page_copy() altogether? I think it's an ugly exception where
the outcome of PageAnon() is not meaningful for an LRU page.
To your patch:
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2856,7 +2856,7 @@ int mem_cgroup_prepare_migration(struct page *page,
>
> /* remove redundant charge if migration failed*/
> void mem_cgroup_end_migration(struct mem_cgroup *mem,
> - struct page *oldpage, struct page *newpage)
> + struct page *oldpage, struct page *newpage, int result)
> {
> struct page *used, *unused;
> struct page_cgroup *pc;
> @@ -2865,8 +2865,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
> return;
> /* blocks rmdir() */
> cgroup_exclude_rmdir(&mem->css);
> - /* at migration success, oldpage->mapping is NULL. */
> - if (oldpage->mapping) {
> + if (result) {
Since this function does not really need more than a boolean value,
wouldn't it make the code more obvious if the parameter was `bool
success'?
if (!success) {
> used = oldpage;
> unused = newpage;
> } else {
Minor nit, though. I agree with the patch in general.
Hannes
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