linux-kernel - Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

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Date:	Fri, 23 Apr 2010 22:54:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Vivek Goyal <vgoyal@...hat.com>, balbir@...ux.vnet.ibm.com,
	Andrea Righi <arighi@...eler.com>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Suleiman Souhlal <suleiman@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

On Fri, 2010-04-23 at 13:17 -0700, Greg Thelen wrote:
> -       lock_page_cgroup(pc);
> +       /*
> +        * Unless a page's cgroup reassignment is possible, then avoid grabbing
> +        * the lock used to protect the cgroup assignment.
> +        */
> +       rcu_read_lock();

Where is the matching barrier?

> +       smp_rmb();
> +       if (unlikely(mem_cgroup_account_move_ongoing)) {
> +               local_irq_save(flags);

So the added irq-disable is a bug-fix?

> +               lock_page_cgroup(pc);
> +               locked = true;
> +       }
> +
>         mem = pc->mem_cgroup;
>         if (!mem || !PageCgroupUsed(pc))
>                 goto done;
> @@ -1449,6 +1468,7 @@ void mem_cgroup_update_file_mapped(struct page *page, int val)
>         /*
>          * Preemption is already disabled. We can use __this_cpu_xxx
>          */
> +       VM_BUG_ON(preemptible());

Insta-bug here, there is nothing guaranteeing we're not preemptible
here.

>         if (val > 0) {
>                 __this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
>                 SetPageCgroupFileMapped(pc);
> @@ -1458,7 +1478,11 @@ void mem_cgroup_update_file_mapped(struct page *page, int val)
>         }
>  
>  done:
> -       unlock_page_cgroup(pc);
> +       if (unlikely(locked)) {
> +               unlock_page_cgroup(pc);
> +               local_irq_restore(flags);
> +       }
> +       rcu_read_unlock();
>  } 

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