Hi,
On Tue, 11 Jan 2011 15:00:50 +0900
Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
> migration itself so charge local variable in unmap_and_move lost the role
> since we introduced 01b1ae63c2.
>
> In addition, the variable name is not good like below.
>
> int unmap_and_move()
> {
> charge = mem_cgroup_prepare_migration(xxx);
> ..
> BUG_ON(charge); <-- BUG if it is charged?
> ..
> uncharge:
> if (!charge) <-- why do we have to uncharge !charge?
> mem_group_end_migration(xxx);
> ..
> }
>
> So let's remove unnecessary and confusing variable.
>
> Suggested-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> Cc: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
> mm/migrate.c | 12 ++++--------
> 1 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b8a32da..e393841 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -623,7 +623,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> struct page *newpage = get_new_page(page, private, &result);
> int remap_swapcache = 1;
> int rcu_locked = 0;
> - int charge = 0;
> struct mem_cgroup *mem = NULL;
> struct anon_vma *anon_vma = NULL;
>
> @@ -662,12 +661,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> }
>
> /* charge against new page */
> - charge = mem_cgroup_prepare_migration(page, newpage, &mem);
> - if (charge == -ENOMEM) {
> - rc = -ENOMEM;
> + rc = mem_cgroup_prepare_migration(page, newpage, &mem);
> + if (rc == -ENOMEM)
> goto unlock;
> - }
> - BUG_ON(charge);
> + BUG_ON(rc);
>
> if (PageWriteback(page)) {
> if (!force || !sync)
> @@ -760,8 +757,7 @@ rcu_unlock:
> if (rcu_locked)
> rcu_read_unlock();
> uncharge:
> - if (!charge)
> - mem_cgroup_end_migration(mem, page, newpage, rc == 0);
> + mem_cgroup_end_migration(mem, page, newpage, rc == 0);
> unlock:
> unlock_page(page);
>
I proposed pseud code like above, but it's wrong unfortunately.
If mem_cgroup_prepare_migration() has succeeded, rc is overwritten to 0.
So even if we failed before calling move_to_new_page(), rc is 0 and
mem_cgroup_end_migration() mis-understand this migration has succeeded.
And, it seems to be just a bit off-topic, the place of the comment
"prepare cgroup just returns 0 or -ENOMEM" isn't good, seeing the commit e8589cc1,
which introduced the comment first.
So, we should do like:
/* charge against new page */
if (mem_cgroup_end_migration(page, &newpage, &mem)) {
/* prepare_migration just returns 0 or -ENOMEM */
rc = -ENOMEM;
goto unlock;
}
if (PageWriteback(page)) {
...
uncharge:
mem_cgroup_end_migration(mem, page, newpage, rc == 0);
or, overwrite rc to -EAGAIN again.
I don't stick to checking "BUG_ON(charge)" personally.
Thanks,
Daisuke Nishimura.
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