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SubjectRe: [RFC][BUGFIX][PATCH 2/2] memcg: fix file mapped underflow at migration (v3)
Hmm, before going further, will you explain why we need a new PCG_MIGRATION flag ?
What's the problem of v2 ?

Thanks,
Daisuke Nishimura.

On Fri, 16 Apr 2010 19:31:43 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> This is still RFC.
> I hope this will fix memcg v.s. page-migraion war finally ;(
> But we have to be careful...
> ==
>
> At page migration, the new page is unlocked before calling end_migration().
> This is mis-understanding with page-migration code of memcg.
> (But it wasn't big problem..the real issue is mapcount handling.)
>
> This has small race and we need to fix this race. By this,
> FILE_MAPPED of migrated file cache is not properly updated, now.
>
> This patch is for fixing the race by changing algorithm.
>
> At migrating mapped file, events happens in following sequence.
>
> 1. allocate a new page.
> 2. get memcg of an old page.
> 3. charge ageinst a new page before migration. But at this point,
> no changes to new page's 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. Here, the new page is unlocked.
> 7. memcg commits the charge for newpage, Mark page cgroup as USED.
>
> Because "commit" happens after page-remap, we cannot count FILE_MAPPED
> at "5", we can lose file_mapped accounting information within this
> small race. FILE_MAPPED is updated only when mapcount changes 0->1.
> So, if we don't catch this 0->1 event, we can underflow FILE_MAPPED
> at 1->0 event.
>
> We may be able to avoid underflow by some small technique or new hook but
> we should catch mapcount 0->1 event fundamentaly. To catch this, we have to
> make page_cgroup of new page as "USED".
>
> BTW, historically, above implemntation comes from migration-failure
> of anonymous page. When we charge both of old page and new page
> with mapcount=0 before migration we can't catch
> - the page is really freed before remap.
> - migration fails but it's freed before remap
> .....corner cases.
>
> For fixing all, this changes parepare/end migration with MIGRATION flag on
> page_cgroup.
>
> New migration sequence with memcg is:
>
> 1. allocate a new page.
> 2. mark PageCgroupMigration to the old page.
> 3. charge against a new page onto the old page's memcg. (here, new page's pc
> is marked as PageCgroupUsed.)
> 4. mark PageCgroupMigration to the new page.
>
> If a page_cgroup is marked as PageCgroupMigration, it's uncahrged until
> it's cleared.
>
> 5. page migration replaces radix-tree, page table, etc...
> 6. At remapping, new page's page_cgroup is now marked as "USED"
> We can catch 0->1 event and FILE_MAPPED will be properly updated.
> 7. Clear PageCgroupMigration of both of old page and new page.
> 8. uncharge unnecessary pages.
>
> By this:
> At migration success of Anon:
> - The new page is properly charged. If not-mapped after remap,
> uncharge() will be called.
> - The file cache is properly charged. FILE_MAPPED event can be caught.
>
> At migration failure of Anon:
> - The old page stays as charged. If not mapped after remap,
> uncharge() will be called after clearing PageCgroupMigration.
> If mapped, or on radix-tree(swapcache), it's not uncharged.
>
> I hope this change will make memcg's page migration much simpler.
> Page migration has caused several troubles. Worth to add a flag
> for simplification.
>
> Changelog: 2010/04/15
> - updated against mm-of-the-moment snapshot 2010-04-15-14-42
> + Nishimura's fix. memcg-fix-prepare-migration-fix.patch
> - fixed some typos.
> - handle migration failure of anon page.
>
> Changelog: 2010/04/14
> - updated onto the latest mmotm + page-compaction, etc...
> - fixed __try_charge() bypass case.
> - use CHARGE_TYPE_FORCE for uncharging an unused page.
>
> Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> include/linux/memcontrol.h | 6 +-
> include/linux/page_cgroup.h | 5 +
> mm/memcontrol.c | 115 +++++++++++++++++++++++++++++---------------
> mm/migrate.c | 2
> 4 files changed, 87 insertions(+), 41 deletions(-)
>
> Index: mmotm-Apr16/mm/memcontrol.c
> ===================================================================
> --- mmotm-Apr16.orig/mm/memcontrol.c
> +++ mmotm-Apr16/mm/memcontrol.c
> @@ -2275,6 +2275,9 @@ __mem_cgroup_uncharge_common(struct page
> if (!PageCgroupUsed(pc))
> goto unlock_out;
>
> + if (unlikely(PageAnon(page) && PageCgroupMigration(pc)))
> + goto unlock_out;
> +
> switch (ctype) {
> case MEM_CGROUP_CHARGE_TYPE_MAPPED:
> case MEM_CGROUP_CHARGE_TYPE_DROP:
> @@ -2501,10 +2504,12 @@ static inline int mem_cgroup_move_swap_a
> * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
> * page belongs to.
> */
> -int mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
> +int mem_cgroup_prepare_migration(struct page *page,
> + struct page *newpage, struct mem_cgroup **ptr)
> {
> struct page_cgroup *pc;
> struct mem_cgroup *mem = NULL;
> + enum charge_type ctype;
> int ret = 0;
>
> if (mem_cgroup_disabled())
> @@ -2515,69 +2520,103 @@ int mem_cgroup_prepare_migration(struct
> if (PageCgroupUsed(pc)) {
> mem = pc->mem_cgroup;
> css_get(&mem->css);
> + /* disallow uncharge until the end of migration */
> + SetPageCgroupMigration(pc);
> }
> unlock_page_cgroup(pc);
> + /*
> + * If the page is uncharged before migration (removed from radix-tree)
> + * we return here.
> + */
> + if (!mem)
> + return 0;
>
> *ptr = mem;
> - if (mem) {
> - ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false);
> - css_put(&mem->css);
> + ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false);
> + css_put(&mem->css);/* drop extra refcnt */
> + if (ret || *ptr == NULL) {
> + lock_page_cgroup(pc);
> + ClearPageCgroupMigration(pc);
> + unlock_page_cgroup(pc);
> + /*
> + * The old page may be fully unmapped while we kept it.
> + * If file cache, we hold lock on this page and there
> + * is no race.
> + */
> + if (PageAnon(page))
> + mem_cgroup_uncharge_page(page);
> + return -ENOMEM;
> + }
> + /*
> + * The old page is under lock_page().
> + * If the old_page is uncharged and freed while migration, page
> + * migration will fail and newpage will properly uncharged.
> + * Because we're only referer to this newpage, this commit_charge
> + * against newpage never fails.
> + */
> + pc = lookup_page_cgroup(newpage);
> + if (PageAnon(page))
> + ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
> + else if (page_is_file_cache(page))
> + ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> + else
> + ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> + __mem_cgroup_commit_charge(mem, pc, ctype);
> + if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED) {
> + lock_page_cgroup(pc);
> + SetPageCgroupMigration(pc);
> + unlock_page_cgroup(pc);
> }
> return ret;
> }
>
> /* remove redundant charge if migration failed*/
> void mem_cgroup_end_migration(struct mem_cgroup *mem,
> - struct page *oldpage, struct page *newpage)
> + struct page *oldpage, struct page *newpage)
> {
> - struct page *target, *unused;
> + struct page *used, *unused;
> struct page_cgroup *pc;
> - enum charge_type ctype;
>
> if (!mem)
> return;
> + /* blocks rmdir() */
> cgroup_exclude_rmdir(&mem->css);
> /* at migration success, oldpage->mapping is NULL. */
> if (oldpage->mapping) {
> - target = oldpage;
> - unused = NULL;
> + used = oldpage;
> + unused = newpage;
> } else {
> - target = newpage;
> + used = newpage;
> unused = oldpage;
> }
> -
> - if (PageAnon(target))
> - ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
> - else if (page_is_file_cache(target))
> - ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> - else
> - ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> -
> - /* unused page is not on radix-tree now. */
> - if (unused)
> - __mem_cgroup_uncharge_common(unused, ctype);
> -
> - pc = lookup_page_cgroup(target);
> /*
> - * __mem_cgroup_commit_charge() check PCG_USED bit of page_cgroup.
> - * So, double-counting is effectively avoided.
> + * We disallowed uncharge of pages under migration because mapcount
> + * of the page goes down to zero, temporarly.
> + * Clear the flag and check the page should be charged.
> */
> - __mem_cgroup_commit_charge(mem, pc, ctype);
> + pc = lookup_page_cgroup(unused);
> + lock_page_cgroup(pc);
> + ClearPageCgroupMigration(pc);
> + unlock_page_cgroup(pc);
> + __mem_cgroup_uncharge_common(unused, MEM_CGROUP_CHARGE_TYPE_FORCE);
>
> + pc = lookup_page_cgroup(used);
> + lock_page_cgroup(pc);
> + ClearPageCgroupMigration(pc);
> + unlock_page_cgroup(pc);
> /*
> - * Both of oldpage and newpage are still under lock_page().
> - * Then, we don't have to care about race in radix-tree.
> - * But we have to be careful that this page is unmapped or not.
> - *
> - * There is a case for !page_mapped(). At the start of
> - * migration, oldpage was mapped. But now, it's zapped.
> - * But we know *target* page is not freed/reused under us.
> - * mem_cgroup_uncharge_page() does all necessary checks.
> - */
> - if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
> - mem_cgroup_uncharge_page(target);
> + * If the page is file cache, radix-tree replacement is very atomic
> + * and we can skip this check. When it comes to Anon pages, it's
> + * uncharged when mapcount goes down to 0. Because page migration
> + * has to make mapcount goes down to 0, we may miss the caes as
> + * migration-failure or really-unmapped-while-migration.
> + * Check it out here.
> + */
> + if (PageAnon(used))
> + mem_cgroup_uncharge_page(used);
> /*
> - * At migration, we may charge account against cgroup which has no tasks
> + * At migration, we may charge account against cgroup which has no
> + * tasks.
> * So, rmdir()->pre_destroy() can be called while we do this charge.
> * In that case, we need to call pre_destroy() again. check it here.
> */
> Index: mmotm-Apr16/mm/migrate.c
> ===================================================================
> --- mmotm-Apr16.orig/mm/migrate.c
> +++ mmotm-Apr16/mm/migrate.c
> @@ -581,7 +581,7 @@ static int unmap_and_move(new_page_t get
> }
>
> /* charge against new page */
> - charge = mem_cgroup_prepare_migration(page, &mem);
> + charge = mem_cgroup_prepare_migration(page, newpage, &mem);
> if (charge == -ENOMEM) {
> rc = -ENOMEM;
> goto unlock;
> Index: mmotm-Apr16/include/linux/memcontrol.h
> ===================================================================
> --- mmotm-Apr16.orig/include/linux/memcontrol.h
> +++ mmotm-Apr16/include/linux/memcontrol.h
> @@ -89,7 +89,8 @@ int mm_match_cgroup(const struct mm_stru
> extern struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *mem);
>
> extern int
> -mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr);
> +mem_cgroup_prepare_migration(struct page *page,
> + struct page *newpage, struct mem_cgroup **ptr);
> extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
> struct page *oldpage, struct page *newpage);
>
> @@ -228,7 +229,8 @@ static inline struct cgroup_subsys_state
> }
>
> static inline int
> -mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
> +mem_cgroup_prepare_migration(struct page *page, struct page *newpage,
> + struct mem_cgroup **ptr)
> {
> return 0;
> }
> Index: mmotm-Apr16/include/linux/page_cgroup.h
> ===================================================================
> --- mmotm-Apr16.orig/include/linux/page_cgroup.h
> +++ mmotm-Apr16/include/linux/page_cgroup.h
> @@ -40,6 +40,7 @@ enum {
> PCG_USED, /* this object is in use. */
> PCG_ACCT_LRU, /* page has been accounted for */
> PCG_FILE_MAPPED, /* page is accounted as "mapped" */
> + PCG_MIGRATION, /* under page migration */
> };
>
> #define TESTPCGFLAG(uname, lname) \
> @@ -79,6 +80,10 @@ SETPCGFLAG(FileMapped, FILE_MAPPED)
> CLEARPCGFLAG(FileMapped, FILE_MAPPED)
> TESTPCGFLAG(FileMapped, FILE_MAPPED)
>
> +SETPCGFLAG(Migration, MIGRATION)
> +CLEARPCGFLAG(Migration, MIGRATION)
> +TESTPCGFLAG(Migration, MIGRATION)
> +
> static inline int page_cgroup_nid(struct page_cgroup *pc)
> {
> return page_to_nid(pc->page);
>


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