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SubjectRe: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat
* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2010-04-13 13:42:07]:

> Hi.
>
> When I was testing page migration, I found underflow problem of "mapped_file" field
> in memory.stat. This is a fix for the problem.
>
> This patch is based on mmotm-2010-04-05-16-09, and IIUC it conflicts with Mel's
> compaction patches, so I send it as RFC for now. After next mmotm, which will
> include those patches, I'll update and resend this patch.
>
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>
> page_add_file_rmap(), which can be called from remove_migration_ptes(), is
> assumed to increment memcg's stat of mapped file. But on success of page
> migration, the newpage(mapped file) has not been charged yet, so the stat will
> not be incremented. This behavior leads to underflow of memcg's stat because
> when the newpage is unmapped afterwards, page_remove_rmap() decrements the stat.
> This problem doesn't happen on failure path of page migration, because the old
> page(mapped file) hasn't been uncharge at the point of remove_migration_ptes().
> This patch fixes this problem by calling commit_charge(mem_cgroup_end_migration)
> before remove_migration_ptes().
>
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
> mm/migrate.c | 19 ++++++++++++++-----
> 1 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 5938db5..915c35e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -485,7 +485,8 @@ static int fallback_migrate_page(struct address_space *mapping,
> * < 0 - error code
> * == 0 - success
> */
> -static int move_to_new_page(struct page *newpage, struct page *page)
> +static int move_to_new_page(struct page *newpage, struct page *page,
> + struct mem_cgroup *mem)
> {
> struct address_space *mapping;
> int rc;
> @@ -520,9 +521,16 @@ static int move_to_new_page(struct page *newpage, struct page *page)
> else
> rc = fallback_migrate_page(mapping, newpage, page);
>
> - if (!rc)
> + if (!rc) {
> + /*
> + * On success of page migration, the newpage has not been
> + * charged yet, so we must call end_migration() before
> + * remove_migration_ptes() to update stats of mapped file
> + * properly.
> + */
> + mem_cgroup_end_migration(mem, page, newpage);
> remove_migration_ptes(page, newpage);
> - else
> + } else
> newpage->mapping = NULL;
>
> unlock_page(newpage);
> @@ -633,7 +641,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>
> skip_unmap:
> if (!page_mapped(page))
> - rc = move_to_new_page(newpage, page);
> + rc = move_to_new_page(newpage, page, mem);

Why do we need to pass mem, won't try_get_mem_cgroup_from_page() help?
Is it cost versus space tradeoff.

>
> if (rc)
> remove_migration_ptes(page, page);
> @@ -641,7 +649,8 @@ rcu_unlock:
> if (rcu_locked)
> rcu_read_unlock();
> uncharge:
> - if (!charge)
> + if (rc)
> + /* On success of page migration, we've alread called it */

Comment is not clear to me, but the code is :)

> mem_cgroup_end_migration(mem, page, newpage);
> unlock:
> unlock_page(page);
> --
> 1.6.4
>
> --
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--
Three Cheers,
Balbir


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