Messages in this thread | | | Date | Tue, 21 Feb 2012 09:55:08 +0400 | From | Konstantin Khlebnikov <> | Subject | Re: [PATCH 6/10] mm/memcg: take care over pc->mem_cgroup |
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Hugh Dickins wrote: > page_relock_lruvec() is using lookup_page_cgroup(page)->mem_cgroup > to find the memcg, and hence its per-zone lruvec for the page. We > therefore need to be careful to see the right pc->mem_cgroup: where > is it updated? > > In __mem_cgroup_commit_charge(), under lruvec lock whenever lru > care might be needed, lrucare holding the page off lru at that time. > > In mem_cgroup_reset_owner(), not under lruvec lock, but before the > page can be visible to others - except compaction or lumpy reclaim, > which ignore the page because it is not yet PageLRU. > > In mem_cgroup_split_huge_fixup(), always under lruvec lock. > > In mem_cgroup_move_account(), which holds several locks, but an > lruvec lock not among them: yet it still appears to be safe, because > the page has been taken off its old lru and not yet put on the new. > > Be particularly careful in compaction's isolate_migratepages() and > vmscan's lumpy handling in isolate_lru_pages(): those approach the > page by its physical location, and so can encounter pages which > would not be found by any logical lookup. For those cases we have > to change __isolate_lru_page() slightly: it must leave ClearPageLRU > to the caller, because compaction and lumpy cannot safely interfere > with a page until they have first isolated it and then locked lruvec. > > To the list above we have to add __mem_cgroup_uncharge_common(), > and new function mem_cgroup_reset_uncharged_to_root(): the first > resetting pc->mem_cgroup to root_mem_cgroup when a page off lru is > uncharged, and the second when an uncharged page is taken off lru > (which used to be achieved implicitly with the PageAcctLRU flag). > > That's because there's a remote risk that compaction or lumpy reclaim > will spy a page while it has PageLRU set; then it's taken off LRU and > freed, its mem_cgroup torn down and freed, the page reallocated (so > get_page_unless_zero again succeeds); then compaction or lumpy reclaim > reach their page_relock_lruvec, using the stale mem_cgroup for locking. > > So long as there's one charge on the mem_cgroup, or a page on one of > its lrus, mem_cgroup_force_empty() cannot succeed and the mem_cgroup > cannot be destroyed. But when an uncharged page is taken off lru, > or a page off lru is uncharged, it no longer protects its old memcg, > and the one stable root_mem_cgroup must then be used for it.
This is much better than my RCU-protected locking. That will be great if it really race-less! I think, I could steal this and polish a little. =)
But just one question: how appears uncharged pages in mem-cg lru lists? Maybe we can forbid this case and uncharge these pages right in __page_cache_release() and release_pages() at final removing from LRU. This is how my old mem-controller works. There pages in lru are always charged.
> > Signed-off-by: Hugh Dickins<hughd@google.com> > --- > include/linux/memcontrol.h | 5 ++ > mm/compaction.c | 36 ++++++----------- > mm/memcontrol.c | 45 +++++++++++++++++++-- > mm/swap.c | 2 > mm/vmscan.c | 73 +++++++++++++++++++++++++---------- > 5 files changed, 114 insertions(+), 47 deletions(-) > > --- mmotm.orig/include/linux/memcontrol.h 2012-02-18 11:57:42.675524592 -0800 > +++ mmotm/include/linux/memcontrol.h 2012-02-18 11:57:49.103524745 -0800 > @@ -65,6 +65,7 @@ extern int mem_cgroup_cache_charge(struc > struct lruvec *mem_cgroup_zone_lruvec(struct zone *, struct mem_cgroup *); > extern struct mem_cgroup *mem_cgroup_from_lruvec(struct lruvec *lruvec); > extern void mem_cgroup_update_lru_size(struct lruvec *, enum lru_list, int); > +extern void mem_cgroup_reset_uncharged_to_root(struct page *); > > /* For coalescing uncharge for reducing memcg' overhead*/ > extern void mem_cgroup_uncharge_start(void); > @@ -251,6 +252,10 @@ static inline void mem_cgroup_update_lru > { > } > > +static inline void mem_cgroup_reset_uncharged_to_root(struct page *page) > +{ > +} > + > static inline struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page) > { > return NULL; > --- mmotm.orig/mm/compaction.c 2012-02-18 11:57:42.675524592 -0800 > +++ mmotm/mm/compaction.c 2012-02-18 11:57:49.103524745 -0800 > @@ -356,28 +356,6 @@ static isolate_migrate_t isolate_migrate > continue; > } > > - if (!lruvec) { > - /* > - * We do need to take the lock before advancing to > - * check PageLRU etc., but there's no guarantee that > - * the page we're peeking at has a stable memcg here. > - */ > - lruvec =&zone->lruvec; > - lock_lruvec(lruvec); > - } > - if (!PageLRU(page)) > - continue; > - > - /* > - * PageLRU is set, and lru_lock excludes isolation, > - * splitting and collapsing (collapsing has already > - * happened if PageLRU is set). > - */ > - if (PageTransHuge(page)) { > - low_pfn += (1<< compound_order(page)) - 1; > - continue; > - } > - > if (!cc->sync) > mode |= ISOLATE_ASYNC_MIGRATE; > > @@ -386,10 +364,24 @@ static isolate_migrate_t isolate_migrate > continue; > > page_relock_lruvec(page,&lruvec); > + if (unlikely(!PageLRU(page) || PageUnevictable(page) || > + PageTransHuge(page))) { > + /* > + * lru_lock excludes splitting a huge page, > + * but we cannot hold lru_lock while freeing page. > + */ > + low_pfn += (1<< compound_order(page)) - 1; > + unlock_lruvec(lruvec); > + lruvec = NULL; > + put_page(page); > + continue; > + } > > VM_BUG_ON(PageTransCompound(page)); > > /* Successfully isolated */ > + ClearPageLRU(page); > + mem_cgroup_reset_uncharged_to_root(page); > del_page_from_lru_list(page, lruvec, page_lru(page)); > list_add(&page->lru, migratelist); > cc->nr_migratepages++; > --- mmotm.orig/mm/memcontrol.c 2012-02-18 11:57:42.679524592 -0800 > +++ mmotm/mm/memcontrol.c 2012-02-18 11:57:49.107524745 -0800 > @@ -1069,6 +1069,33 @@ void page_relock_lruvec(struct page *pag > *lruvp = lruvec; > } > > +void mem_cgroup_reset_uncharged_to_root(struct page *page) > +{ > + struct page_cgroup *pc; > + > + if (mem_cgroup_disabled()) > + return; > + > + VM_BUG_ON(PageLRU(page)); > + > + /* > + * Once an uncharged page is isolated from the mem_cgroup's lru, > + * it no longer protects that mem_cgroup from rmdir: reset to root. > + * > + * __page_cache_release() and release_pages() may be called at > + * interrupt time: we cannot lock_page_cgroup() then (we might > + * have interrupted a section with page_cgroup already locked), > + * nor do we need to since the page is frozen and about to be freed. > + */ > + pc = lookup_page_cgroup(page); > + if (page_count(page)) > + lock_page_cgroup(pc); > + if (!PageCgroupUsed(pc)&& pc->mem_cgroup != root_mem_cgroup) > + pc->mem_cgroup = root_mem_cgroup; > + if (page_count(page)) > + unlock_page_cgroup(pc); > +} > + > /** > * mem_cgroup_update_lru_size - account for adding or removing an lru page > * @lruvec: mem_cgroup per zone lru vector > @@ -2865,6 +2892,7 @@ __mem_cgroup_uncharge_common(struct page > struct mem_cgroup *memcg = NULL; > unsigned int nr_pages = 1; > struct page_cgroup *pc; > + struct lruvec *lruvec; > bool anon; > > if (mem_cgroup_disabled()) > @@ -2884,6 +2912,7 @@ __mem_cgroup_uncharge_common(struct page > if (unlikely(!PageCgroupUsed(pc))) > return NULL; > > + lruvec = page_lock_lruvec(page); > lock_page_cgroup(pc); > > memcg = pc->mem_cgroup; > @@ -2915,14 +2944,17 @@ __mem_cgroup_uncharge_common(struct page > mem_cgroup_charge_statistics(memcg, anon, -nr_pages); > > ClearPageCgroupUsed(pc); > + > /* > - * pc->mem_cgroup is not cleared here. It will be accessed when it's > - * freed from LRU. This is safe because uncharged page is expected not > - * to be reused (freed soon). Exception is SwapCache, it's handled by > - * special functions. > + * Once an uncharged page is isolated from the mem_cgroup's lru, > + * it no longer protects that mem_cgroup from rmdir: reset to root. > */ > + if (!PageLRU(page)&& pc->mem_cgroup != root_mem_cgroup) > + pc->mem_cgroup = root_mem_cgroup; > > unlock_page_cgroup(pc); > + unlock_lruvec(lruvec); > + > /* > * even after unlock, we have memcg->res.usage here and this memcg > * will never be freed. > @@ -2939,6 +2971,7 @@ __mem_cgroup_uncharge_common(struct page > > unlock_out: > unlock_page_cgroup(pc); > + unlock_lruvec(lruvec); > return NULL; > } > > @@ -3327,7 +3360,9 @@ static struct page_cgroup *lookup_page_c > * the first time, i.e. during boot or memory hotplug; > * or when mem_cgroup_disabled(). > */ > - if (likely(pc)&& PageCgroupUsed(pc)) > + if (!pc || PageCgroupUsed(pc)) > + return pc; > + if (pc->mem_cgroup&& pc->mem_cgroup != root_mem_cgroup) > return pc; > return NULL; > } > --- mmotm.orig/mm/swap.c 2012-02-18 11:57:42.679524592 -0800 > +++ mmotm/mm/swap.c 2012-02-18 11:57:49.107524745 -0800 > @@ -52,6 +52,7 @@ static void __page_cache_release(struct > lruvec = page_lock_lruvec(page); > VM_BUG_ON(!PageLRU(page)); > __ClearPageLRU(page); > + mem_cgroup_reset_uncharged_to_root(page); > del_page_from_lru_list(page, lruvec, page_off_lru(page)); > unlock_lruvec(lruvec); > } > @@ -583,6 +584,7 @@ void release_pages(struct page **pages, > page_relock_lruvec(page,&lruvec); > VM_BUG_ON(!PageLRU(page)); > __ClearPageLRU(page); > + mem_cgroup_reset_uncharged_to_root(page); > del_page_from_lru_list(page, lruvec, page_off_lru(page)); > } > > --- mmotm.orig/mm/vmscan.c 2012-02-18 11:57:42.679524592 -0800 > +++ mmotm/mm/vmscan.c 2012-02-18 11:57:49.107524745 -0800 > @@ -1087,11 +1087,11 @@ int __isolate_lru_page(struct page *page > > if (likely(get_page_unless_zero(page))) { > /* > - * Be careful not to clear PageLRU until after we're > - * sure the page is not being freed elsewhere -- the > - * page release code relies on it. > + * Beware of interface change: now leave ClearPageLRU(page) > + * to the caller, because memcg's lumpy and compaction > + * cases (approaching the page by its physical location) > + * may not have the right lru_lock yet. > */ > - ClearPageLRU(page); > ret = 0; > } > > @@ -1154,7 +1154,16 @@ static unsigned long isolate_lru_pages(u > > switch (__isolate_lru_page(page, mode, file)) { > case 0: > +#ifdef CONFIG_DEBUG_VM > + /* check lock on page is lock we already got */ > + page_relock_lruvec(page,&lruvec); > + BUG_ON(lruvec != home_lruvec); > + BUG_ON(page != lru_to_page(src)); > + BUG_ON(page_lru(page) != lru); > +#endif > + ClearPageLRU(page); > isolated_pages = hpage_nr_pages(page); > + mem_cgroup_reset_uncharged_to_root(page); > mem_cgroup_update_lru_size(lruvec, lru, -isolated_pages); > list_move(&page->lru, dst); > nr_taken += isolated_pages; > @@ -1211,21 +1220,7 @@ static unsigned long isolate_lru_pages(u > !PageSwapCache(cursor_page)) > break; > > - if (__isolate_lru_page(cursor_page, mode, file) == 0) { > - mem_cgroup_page_relock_lruvec(cursor_page, > -&lruvec); > - isolated_pages = hpage_nr_pages(cursor_page); > - mem_cgroup_update_lru_size(lruvec, > - page_lru(cursor_page), -isolated_pages); > - list_move(&cursor_page->lru, dst); > - > - nr_taken += isolated_pages; > - nr_lumpy_taken += isolated_pages; > - if (PageDirty(cursor_page)) > - nr_lumpy_dirty += isolated_pages; > - scan++; > - pfn += isolated_pages - 1; > - } else { > + if (__isolate_lru_page(cursor_page, mode, file) != 0) { > /* > * Check if the page is freed already. > * > @@ -1243,13 +1238,50 @@ static unsigned long isolate_lru_pages(u > continue; > break; > } > + > + /* > + * This locking call is a no-op in the non-memcg > + * case, since we already hold the right lru_lock; > + * but it may change the lock in the memcg case. > + * It is then vital to recheck PageLRU (but not > + * necessary to recheck isolation mode). > + */ > + mem_cgroup_page_relock_lruvec(cursor_page,&lruvec); > + > + if (PageLRU(cursor_page)&& > + !PageUnevictable(cursor_page)) { > + ClearPageLRU(cursor_page); > + isolated_pages = hpage_nr_pages(cursor_page); > + mem_cgroup_reset_uncharged_to_root(cursor_page); > + mem_cgroup_update_lru_size(lruvec, > + page_lru(cursor_page), -isolated_pages); > + list_move(&cursor_page->lru, dst); > + > + nr_taken += isolated_pages; > + nr_lumpy_taken += isolated_pages; > + if (PageDirty(cursor_page)) > + nr_lumpy_dirty += isolated_pages; > + scan++; > + pfn += isolated_pages - 1; > + } else { > + /* Cannot hold lru_lock while freeing page */ > + unlock_lruvec(lruvec); > + lruvec = NULL; > + put_page(cursor_page); > + break; > + } > } > > /* If we break out of the loop above, lumpy reclaim failed */ > if (pfn< end_pfn) > nr_lumpy_failed++; > > - lruvec = home_lruvec; > + if (lruvec != home_lruvec) { > + if (lruvec) > + unlock_lruvec(lruvec); > + lruvec = home_lruvec; > + lock_lruvec(lruvec); > + } > } > > *nr_scanned = scan; > @@ -1301,6 +1333,7 @@ int isolate_lru_page(struct page *page) > int lru = page_lru(page); > get_page(page); > ClearPageLRU(page); > + mem_cgroup_reset_uncharged_to_root(page); > del_page_from_lru_list(page, lruvec, lru); > ret = 0; > }
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