Messages in this thread | | | Date | Sat, 27 Sep 2008 13:26:17 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH 7/12] memcg add function to move account |
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KAMEZAWA Hiroyuki wrote: > This patch provides a function to move account information of a page between > mem_cgroups. >
What is the interface for moving the accounting? Is it an explicit call like force_empty? The other concern I have is about merging two LRU lists, when we move LRU pages from one mem_cgroup to another, where do we add them? To the head or tail? I think we need to think about it and document it well.
The other thing is that once we have mult-hierarchy support (which we really need), we need to move the accounting to the parent instead of root.
> This moving of page_cgroup is done under > - lru_lock of source/destination mem_cgroup is held.
I suppose you mean and instead of either for the lru_lock
> - lock_page_cgroup() is held. > > Then, a routine which touches pc->mem_cgroup without lock_page_cgroup() should > confirm pc->mem_cgroup is still valid or not. Typlical code can be following. > > (while page is not under lock_page()) > mem = pc->mem_cgroup; > mz = page_cgroup_zoneinfo(pc) > spin_lock_irqsave(&mz->lru_lock); > if (pc->mem_cgroup == mem) > ...../* some list handling */ > spin_unlock_irq(&mz->lru_lock); > > Or better way is > lock_page_cgroup(pc); > .... > unlock_page_cgroup(pc); > > But you should confirm the nest of lock and avoid deadlock. > (trylock is better if it's ok.) > > If you find page_cgroup from mem_cgroup's LRU under mz->lru_lock, > you don't have to worry about what pc->mem_cgroup points to. > > Changelog: (v4) -> (v5) > - check for lock_page() is removed. > - rewrote description. > > Changelog: (v2) -> (v4) > - added lock_page_cgroup(). > - splitted out from new-force-empty patch. > - added how-to-use text. > - fixed race in __mem_cgroup_uncharge_common(). > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > mm/memcontrol.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 81 insertions(+), 3 deletions(-) > > Index: mmotm-2.6.27-rc7+/mm/memcontrol.c > =================================================================== > --- mmotm-2.6.27-rc7+.orig/mm/memcontrol.c > +++ mmotm-2.6.27-rc7+/mm/memcontrol.c > @@ -426,6 +426,7 @@ int task_in_mem_cgroup(struct task_struc > void mem_cgroup_move_lists(struct page *page, enum lru_list lru) > { > struct page_cgroup *pc; > + struct mem_cgroup *mem; > struct mem_cgroup_per_zone *mz; > unsigned long flags; > > @@ -444,9 +445,14 @@ void mem_cgroup_move_lists(struct page * > > pc = page_get_page_cgroup(page); > if (pc) { > + mem = pc->mem_cgroup; > mz = page_cgroup_zoneinfo(pc); > spin_lock_irqsave(&mz->lru_lock, flags); > - __mem_cgroup_move_lists(pc, lru); > + /* > + * check against the race with move_account. > + */ > + if (likely(mem == pc->mem_cgroup)) > + __mem_cgroup_move_lists(pc, lru); > spin_unlock_irqrestore(&mz->lru_lock, flags); > } > unlock_page_cgroup(page); > @@ -567,6 +573,70 @@ unsigned long mem_cgroup_isolate_pages(u > return nr_taken; > } > > +/** > + * mem_cgroup_move_account - move account of the page > + * @page ... the target page of being moved. > + * @pc ... page_cgroup of the page. > + * @from ... mem_cgroup which the page is moved from. > + * @to ... mem_cgroup which the page is moved to. > + * > + * The caller must confirm following. > + * 1. disable irq. > + * 2. lru_lock of old mem_cgroup should be held. > + * 3. pc is guaranteed to be valid and on mem_cgroup's LRU. > + * > + * Because we cannot call try_to_free_page() here, the caller must guarantee > + * this moving of charge never fails. (if charge fails, this call fails.) > + * Currently this is called only against root cgroup. > + * which has no limitation of resource. > + * Returns 0 at success, returns 1 at failure. > + */ > +int mem_cgroup_move_account(struct page *page, struct page_cgroup *pc, > + struct mem_cgroup *from, struct mem_cgroup *to) > +{ > + struct mem_cgroup_per_zone *from_mz, *to_mz; > + int nid, zid; > + int ret = 1; > + > + VM_BUG_ON(!irqs_disabled()); > + > + nid = page_to_nid(page); > + zid = page_zonenum(page); > + from_mz = mem_cgroup_zoneinfo(from, nid, zid); > + to_mz = mem_cgroup_zoneinfo(to, nid, zid); > + > + if (res_counter_charge(&to->res, PAGE_SIZE)) { > + /* Now, we assume no_limit...no failure here. */ > + return ret; > + }
Please BUG_ON() if the charging fails, we can be sure we catch assumptions that are broken.
> + if (!try_lock_page_cgroup(page)) { > + res_counter_uncharge(&to->res, PAGE_SIZE); > + return ret; > + } > + > + if (page_get_page_cgroup(page) != pc) { > + res_counter_uncharge(&to->res, PAGE_SIZE); > + goto out; > + } > + > + if (spin_trylock(&to_mz->lru_lock)) {
The spin_trylock is to avoid deadlocks, right?
> + __mem_cgroup_remove_list(from_mz, pc); > + css_put(&from->css); > + res_counter_uncharge(&from->res, PAGE_SIZE); > + pc->mem_cgroup = to; > + css_get(&to->css); > + __mem_cgroup_add_list(to_mz, pc); > + ret = 0; > + spin_unlock(&to_mz->lru_lock); > + } else { > + res_counter_uncharge(&to->res, PAGE_SIZE); > + } > +out: > + unlock_page_cgroup(page); > + > + return ret; > +} > + > /* > * Charge the memory controller for page usage. > * Return > @@ -754,16 +824,24 @@ __mem_cgroup_uncharge_common(struct page > if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED) > && ((PageCgroupCache(pc) || page_mapped(page)))) > goto unlock; > - > +retry: > + mem = pc->mem_cgroup; > mz = page_cgroup_zoneinfo(pc); > spin_lock_irqsave(&mz->lru_lock, flags); > + if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED && > + unlikely(mem != pc->mem_cgroup)) { > + /* MAPPED account can be done without lock_page(). > + Check race with mem_cgroup_move_account() */
Coding style above is broken. Can this race really occur? Why do we get mem before acquiring the mz->lru_lock? We don't seem to be using it.
> + spin_unlock_irqrestore(&mz->lru_lock, flags); > + goto retry; > + } > __mem_cgroup_remove_list(mz, pc); > spin_unlock_irqrestore(&mz->lru_lock, flags); > > page_assign_page_cgroup(page, NULL); > unlock_page_cgroup(page); > > - mem = pc->mem_cgroup; > + > res_counter_uncharge(&mem->res, PAGE_SIZE); > css_put(&mem->css); > >
-- Balbir
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