On Wed, 5 Jan 2011 12:58:40 +0100
Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> 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?
>
As long as I can see, no.
> 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.
>
IIUC, oldpage will be freed on success of page migration, so we hit bad_page
check at freeing the page unless we clear oldpage->mapping,
> 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.
>
Thank you for your review.
How about this ?
===
From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
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.
Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
---
include/linux/memcontrol.h | 5 ++---
mm/memcontrol.c | 5 ++---
mm/migrate.c | 2 +-
3 files changed, 5 insertions(+), 7 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 159a076..cc5a8fd 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -93,7 +93,7 @@ extern int
mem_cgroup_prepare_migration(struct page *page,
struct page *newpage, struct mem_cgroup **ptr);
extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
- struct page *oldpage, struct page *newpage);
+ struct page *oldpage, struct page *newpage, bool success);
/*
* For memory reclaim.
@@ -231,8 +231,7 @@ mem_cgroup_prepare_migration(struct page *page, struct page *newpage,
}
static inline void mem_cgroup_end_migration(struct mem_cgroup *mem,
- struct page *oldpage,
- struct page *newpage)
+ struct page *oldpage, struct page *newpage, bool success)
{
}
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 61678be..fbecd02 100644
--- 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, bool success)
{
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 (!success) {
used = oldpage;
unused = newpage;
} else {
diff --git a/mm/migrate.c b/mm/migrate.c
index 6ae8a66..be66b23 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -756,7 +756,7 @@ rcu_unlock:
rcu_read_unlock();
uncharge:
if (!charge)
- mem_cgroup_end_migration(mem, page, newpage);
+ mem_cgroup_end_migration(mem, page, newpage, rc == 0);
unlock:
unlock_page(page);
--
1.7.1
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