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Date: Thu, 23 Jun 2011 16:23:21 +0900
From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Michal Hocko <mhocko@...e.cz>,
Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Rik van Riel <riel@...hat.com>,
Michel Lespinasse <walken@...gle.com>,
Mel Gorman <mgorman@...e.de>, Lutz Vieweg <lvml@....de>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: unlock page before charging it. (WasRe: [PATCH
V2] mm: Do not keep page locked during page fault while charging it for
memcg
On Thu, 23 Jun 2011 15:08:42 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 22 Jun 2011 14:32:04 +0200
> Michal Hocko <mhocko@...e.cz> wrote:
>
> > On Wed 22-06-11 08:15:16, Christoph Hellwig wrote:
> > > > +
> > > > + /* We have to drop the page lock here because memcg
> > > > + * charging might block for unbound time if memcg oom
> > > > + * killer is disabled.
> > > > + */
> > > > + unlock_page(vmf.page);
> > > > + ret = mem_cgroup_newpage_charge(page, mm, GFP_KERNEL);
> > > > + lock_page(vmf.page);
> > >
> > > This introduces a completely poinless unlock/lock cycle for non-memcg
> > > pagefaults. Please make sure it only happens when actually needed.
> >
> > Fair point. Thanks!
> > What about the following?
> > I realize that pushing more memcg logic into mm/memory.c is not nice but
> > I found it better than pushing the old page into mem_cgroup_newpage_charge.
> > We could also check whether the old page is in the root cgroup because
> > memcg oom killer is not active there but that would add more code into
> > this hot path so I guess it is not worth it.
> >
> > Changes since v1
> > - do not unlock page when memory controller is disabled.
> >
>
> Great work. Then I confirmed Lutz' problem is fixed.
>
> But I like following style rather than additional lock/unlock.
> How do you think ? I tested this on the latest git tree and confirmed
> the Lutz's livelock problem is fixed. And I think this should go stable tree.
>
I vote for this one.
One comments are inlined below.
>
> ==
> From 7e9250da9ff529958d4c1ff511458dbdac8e4b81 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Thu, 23 Jun 2011 15:05:57 +0900
> Subject: [PATCH] memcg: unlock page before charging it.
>
> Currently we are keeping faulted page locked throughout whole __do_fault
> call (except for page_mkwrite code path). If we do early COW we allocate a
> new page which has to be charged for a memcg (mem_cgroup_newpage_charge).
>
> This function, however, might block for unbounded amount of time if memcg
> oom killer is disabled or fork-bomb is running because the only way out of
> the OOM situation is either an external event or OOM-situation fix.
>
> processes from faulting it in which is not good at all because we are
> basically punishing potentially an unrelated process for OOM condition
> in a different group (I have seen stuck system because of ld-2.11.1.so being
> locked).
>
> We can do test easily.
> % cgcreate -g memory:A
> % cgset -r memory.limit_in_bytes=64M A
> % cgset -r memory.memsw.limit_in_bytes=64M A
> % cd kernel_dir; cgexec -g memory:A make -j
>
> Then, the whole system will live-locked until you kill 'make -j'
> by hands (or push reboot...) This is because some important
> page in a shared library are locked and never released bcause of fork-bomb.
>
> This patch delays "charge" until unlock_page() called. There is
> no problem as far as we keep reference on a page.
> (memcg doesn't require page_lock()).
>
> Then, above livelock disappears.
>
> Reported-by: Lutz Vieweg <lvml@....de>
> Original-idea-by: Michal Hocko <mhocko@...e.cz>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> mm/memory.c | 28 +++++++++++++++++++---------
> 1 files changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 87d9353..66442da 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3129,7 +3129,7 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> struct page *page;
> pte_t entry;
> int anon = 0;
> - int charged = 0;
> + struct page *need_charge = NULL;
> struct page *dirty_page = NULL;
> struct vm_fault vmf;
> int ret;
> @@ -3177,12 +3177,7 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> ret = VM_FAULT_OOM;
> goto out;
> }
> - if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) {
> - ret = VM_FAULT_OOM;
> - page_cache_release(page);
> - goto out;
> - }
> - charged = 1;
> + need_charge = page;
> copy_user_highpage(page, vmf.page, address, vma);
> __SetPageUptodate(page);
> } else {
> @@ -3251,12 +3246,11 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> /* no need to invalidate: a not-present page won't be cached */
> update_mmu_cache(vma, address, page_table);
> } else {
> - if (charged)
> - mem_cgroup_uncharge_page(page);
> if (anon)
> page_cache_release(page);
> else
> anon = 1; /* no anon but release faulted_page */
> + need_charge = NULL;
> }
>
> pte_unmap_unlock(page_table, ptl);
> @@ -3268,6 +3262,17 @@ out:
> if (set_page_dirty(dirty_page))
> page_mkwrite = 1;
> unlock_page(dirty_page);
> + if (need_charge) {
> + /*
> + * charge this page before we drop refcnt.
> + * memory cgroup returns OOM condition when
> + * this task is killed. So, it's not necesasry
> + * to undo.
> + */
> + if (mem_cgroup_newpage_charge(need_charge,
> + mm, GFP_KERNEL))
> + ret = VM_FAULT_OOM;
> + }
> put_page(dirty_page);
> if (page_mkwrite && mapping) {
> /*
Hmm, if I read the code correctly, we don't come to this path.
Because "dirty_page" is set only in "anon == 0" case and, when we set "need_charge",
we set "anon" too.
So, we can do mem_cgroup_newpage_charge(need_charge) outside of
"if (dirty_page) ... else ..." block ?
Thanks,
Daisuke Nishimura.
> @@ -3282,6 +3287,11 @@ out:
> file_update_time(vma->vm_file);
> } else {
> unlock_page(vmf.page);
> + if (need_charge) {
> + if (mem_cgroup_newpage_charge(need_charge,
> + mm, GFP_KERNEL))
> + ret = VM_FAULT_OOM;
> + }
> if (anon)
> page_cache_release(vmf.page);
> }
> --
> 1.7.4.1
>
>
>
>
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