Messages in this thread Patch in this message | | | Date | Tue, 13 Apr 2010 15:14:00 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat |
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On Tue, 13 Apr 2010 13:42:07 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> 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@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>
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@mxp.nes.nec.co.jp> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> --- 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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