On Tue, 13 Apr 2010 13:42:07 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxx>
Nice catch. but...I want to make all kind of complicated things under
prepare/end migration. (And I want to avoid changes in migrate.c...)
Considering some racy condistions, I wonder memcg_update_file_mapped() itself
still need fixes..
So, how about this ? We already added FILE_MAPPED flags, then, make use of it.
==
At migrating mapped file, events happens in following sequence.
1. allocate a new page.
2. get memcg of an old page.
3. charge ageinst new page, before migration. But at this point
no changes to page_cgroup, no commit-charge.
4. page migration replaces radix-tree, old-page and new-page.
5. page migration remaps the new page if the old page was mapped.
6. memcg commits the charge for newpage.
Because "commit" happens after page-remap, we lose file_mapped
accounting information at migration.
This patch fixes it by accounting file_mapped information at
commiting charge.
Reported-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
mm/memcontrol.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
Index: mmotm-temp/mm/memcontrol.c
===================================================================
--- mmotm-temp.orig/mm/memcontrol.c
+++ mmotm-temp/mm/memcontrol.c
@@ -1435,11 +1435,13 @@ void mem_cgroup_update_file_mapped(struc
/*
* Preemption is already disabled. We can use __this_cpu_xxx
+ * We have no lock per page at inc/dec mapcount of pages. We have to do
+ * check by ourselves under lock_page_cgroup().
*/
- if (val > 0) {
+ if (val > 0 && !PageCgroupFileMapped(pc)) {
__this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
SetPageCgroupFileMapped(pc);
- } else {
+ } else if (PageCgroupFileMapped(pc)) {
__this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
ClearPageCgroupFileMapped(pc);
}
@@ -2563,6 +2565,15 @@ void mem_cgroup_end_migration(struct mem
*/
if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
mem_cgroup_uncharge_page(target);
+ else {
+ /*
+ * When a migrated file cache is remapped, it's not charged.
+ * Verify it. Because we're under lock_page(), there are
+ * no race with uncharge.
+ */
+ if (page_mapped(target))
+ mem_cgroup_update_file_mapped(mem, target, 1);
+ }
/*
* At migration, we may charge account against cgroup which has no tasks
* So, rmdir()->pre_destroy() can be called while we do this charge.
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