On Mon, Feb 21, 2011 at 12:17:18AM +0900, Minchan Kim 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.
>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> Suggested-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> Acked-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> ---
> mm/memcontrol.c | 1 +
> mm/migrate.c | 9 +++------
> 2 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8a97571..3c91d5c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2873,6 +2873,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
> /*
> * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
> * page belongs to.
> + * Note: Should not return -EAGAIN. unmap_and_move depens on it.
> */
> int mem_cgroup_prepare_migration(struct page *page,
> struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 2abc9c9..37055d0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> int *result = NULL;
> struct page *newpage = get_new_page(page, private, &result);
> int remap_swapcache = 1;
> - int charge = 0;
> struct mem_cgroup *mem;
> struct anon_vma *anon_vma = NULL;
>
> @@ -637,7 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> if (unlikely(split_huge_page(page)))
> goto move_newpage;
>
> - /* prepare cgroup just returns 0 or -ENOMEM */
> + /* mem_cgroup_prepage_migration never returns -EAGAIN */
> rc = -EAGAIN;
I really don't like this. Why should we depend on that?
> if (!trylock_page(page)) {
> @@ -678,8 +677,7 @@ 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, GFP_KERNEL);
> - if (charge == -ENOMEM) {
> + if (mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL)) {
> rc = -ENOMEM;
> goto unlock;
Couldn't we make unmap_and_move completely oblivious of the specific
value and just do
rc = mem_cgroup_prepare_migration()
if (rc)
goto unlock;
at this point? I think mem_cgroup_prepare_migration should be rather
free to signal pretty much any error and it is up to migrate_pages()
to handle them correctly.
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