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Message-Id: <20111221190801.5b10e80c.kamezawa.hiroyu@jp.fujitsu.com>
Date: Wed, 21 Dec 2011 19:08:01 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>, cgroups@...r.kernel.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] memcg: return -EINTR at bypassing try_charge().
On Wed, 21 Dec 2011 10:57:08 +0100
Johannes Weiner <hannes@...xchg.org> wrote:
> On Wed, Dec 21, 2011 at 05:24:23PM +0900, KAMEZAWA Hiroyuki wrote:
> > How about this ?
> > --
> > >From 6076425613f594d442c58a5d463c09f8309236aa Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Date: Wed, 21 Dec 2011 16:27:25 +0900
> > Subject: [PATCH] memcg: return -EINTR at bypassing try_charge().
> >
> > This patch is a fix for memcg-simplify-lru-handling-by-new-rule.patch
> > When running testprogram and stop it by Ctrl-C, add_lru/del_lru
> > will find pc->mem_cgroup is NULL and get panic. The reason
> > is bypass code in try_charge().
> >
> > At try_charge(), it checks the thread is fatal or not as..
> > fatal_signal_pending() or TIF_MEMDIE. In this case, __try_charge()
> > returns 0(success) with setting *ptr as NULL.
> >
> > Now, lruvec are deteremined by pc->mem_cgroup. So, it's better
> > to reset pc->mem_cgroup as root_mem_cgroup. This patch does
> > following change in try_charge()
> > 1. return -EINTR at bypassing.
> > 2. set *ptr = root_mem_cgroup at bypassing.
> >
> > By this change, in page fault / radix-tree-insert path,
> > the page will be charged against root_mem_cgroup and the thread's
> > operations will go ahead without trouble. In other path,
> > migration or move_account etc..., -EINTR will stop the operation.
> > (may need some cleanup later..)
> >
> > After this change, pc->mem_cgroup will have valid pointer if
> > the page is used.
> >
> > Changelog: v1 -> v2
> > - returns -EINTR at bypassing.
> > - change error code handling at callers.
> > - changed the name of patch.
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > mm/memcontrol.c | 53 +++++++++++++++++++++++++++++++++++++++++------------
> > 1 files changed, 41 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 9175097..3c6eb7e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2185,6 +2185,23 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > }
> >
> > /*
> > + * __mem_cgroup_try_charge() does
> > + * 1. detect memcg to be charged against from passed *mm and *ptr,
> > + * 2. update res_counter
> > + * 3. call memory reclaim if necessary.
> > + *
> > + * In some special case, if the task is fatal, fatal_signal_pending() or
> > + * TIF_MEMDIE or ->mm is NULL, this functoion returns -EINTR with filling
> > + * *ptr as root_mem_cgroup. There are 2 reasons for this. 1st is that
> > + * fatal threads should quit as soon as possible without any hazards.
> > + * 2nd is that all page should have valid pc->mem_cgroup if it will be
> > + * used.
> > + *
> > + * So, try_charge will return
> > + * 0 ... at success. filling *ptr with a valid memcg pointer.
> > + * -ENOMEM ... charge failure because of resource limits.
> > + * -EINTR ... if thread is fatal. *ptr is filled with root_mem_cgroup.
> > + *
> > * Unlike exported interface, "oom" parameter is added. if oom==true,
> > * oom-killer can be invoked.
> > */
> > @@ -2316,8 +2333,8 @@ nomem:
> > *ptr = NULL;
> > return -ENOMEM;
> > bypass:
> > - *ptr = NULL;
> > - return 0;
> > + *ptr = root_mem_cgroup;
> > + return -EINTR;
> > }
>
> What about this case:
>
> /*
> * We always charge the cgroup the mm_struct belongs to.
> * The mm_struct's mem_cgroup changes on task migration if the
> * thread group leader migrates. It's possible that mm is not
> * set, if so charge the init_mm (happens for pagecache usage).
> */
> if (!*ptr && !mm)
> goto bypass;
>
IIUC, task->mm is NULL when the task is about to exit.
So, charge to root_mem_cgroup will be enough good.
I'm not sure how 'mm_struct's mem_cgroup changes on.....' cases affect
this path...
> > @@ -2564,7 +2581,7 @@ static int mem_cgroup_move_parent(struct page *page,
> > {
> > struct cgroup *cg = child->css.cgroup;
> > struct cgroup *pcg = cg->parent;
> > - struct mem_cgroup *parent;
> > + struct mem_cgroup *parent, *ptr;
> > unsigned int nr_pages;
> > unsigned long uninitialized_var(flags);
> > int ret;
> > @@ -2582,8 +2599,8 @@ static int mem_cgroup_move_parent(struct page *page,
> > nr_pages = hpage_nr_pages(page);
> >
> > parent = mem_cgroup_from_cont(pcg);
> > - ret = __mem_cgroup_try_charge(NULL, gfp_mask, nr_pages, &parent, false);
> > - if (ret || !parent)
> > + ret = __mem_cgroup_try_charge(NULL, gfp_mask, nr_pages, &ptr, false);
> > + if (ret)
> > goto put_back;
>
> That doesn't seem right. That unitilialized ptr is used in
> try_charge(), so this may crash, and it should really charge against
> parent.
Ah, yes. my mistake.
> > @@ -2630,9 +2647,9 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
> >
> > pc = lookup_page_cgroup(page);
> > ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
> > - if (ret || !memcg)
> > + if (ret == -ENOMEM)
> > return ret;
> > -
> > + /* we'll bypass -EINTR case and charge this page to root memcg */
> > __mem_cgroup_commit_charge(memcg, page, nr_pages, pc, ctype);
> > return 0;
> > }
>
> This comment is not very useful. WHY do we do this? Maybe just copy
> the comment from try_charge_swapin()?
>
Hmm, ok. I'll remove this comment.
> > @@ -2703,6 +2720,7 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
> > ret = mem_cgroup_charge_common(page, mm, gfp_mask, type);
> > else { /* page is swapcache/shmem */
> > ret = mem_cgroup_try_charge_swapin(mm, page, gfp_mask, &memcg);
> > + /* see try_charge_swapi() for -EINTR case */
> > if (!ret)
> > __mem_cgroup_commit_charge_swapin(page, memcg, type);
> > }
>
> Missing n in the comment.
>
will remove this, too.
> > @@ -2743,11 +2761,21 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
> > *memcgp = memcg;
> > ret = __mem_cgroup_try_charge(NULL, mask, 1, memcgp, true);
> > css_put(&memcg->css);
> > + /*
> > + * If this thread is fatal, charge against root cgroup and allow
> > + * this thread to exit in quick manner. EINTR is not handled
> > + * in page fault path. So, just bypass this.
> > + */
> > + if (ret == -EINTR)
> > + ret = 0;
> > return ret;
> > charge_cur_mm:
> > if (unlikely(!mm))
> > mm = &init_mm;
> > - return __mem_cgroup_try_charge(mm, mask, 1, memcgp, true);
> > + ret = __mem_cgroup_try_charge(mm, mask, 1, memcgp, true);
> > + if (ret == -EINTR)
> > + ret = 0;
> > + return ret;
> > }
> >
> > static void
>
> > @@ -3633,7 +3662,7 @@ static int mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
> > pc = lookup_page_cgroup(page);
> >
> > ret = mem_cgroup_move_parent(page, pc, memcg, GFP_KERNEL);
> > - if (ret == -ENOMEM)
> > + if (ret == -ENOMEM || ret == -EINTR)
> > break;
>
> if (ret)
>
-EBUSY check is below this code.
I'll prepare v3.
Thanks,
-Kame
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