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Message-ID: <48DDE721.9060309@linux.vnet.ibm.com>
Date: Sat, 27 Sep 2008 13:26:17 +0530
From: Balbir Singh <balbir@...ux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"xemul@...nvz.org" <xemul@...nvz.org>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Dave Hansen <haveblue@...ibm.com>, ryov@...inux.co.jp
Subject: Re: [PATCH 7/12] memcg add function to move account
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@...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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