Re: [PATCH 1/2] memcg: flatten task_struct->memcg_oom

From: Andrey Ryabinin <ryabinin.a.a@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Sasha Levin <sasha.levin@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	mhocko@kernel.org, cgroups@vger.kernel.org,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	vdavydov@parallels.com, kernel-team@fb.com,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH 1/2] memcg: flatten task_struct->memcg_oom
Date: Wed, 25 Nov 2015 18:31:41 +0300	[thread overview]
Message-ID: <CAPAsAGwa9-7UBUnhysfek3kyWKMgaUJRwtDPEqas1rKwkeTtoA@mail.gmail.com> (raw)
In-Reply-To: <20151125150207.GM11639@twins.programming.kicks-ass.net>

2015-11-25 18:02 GMT+03:00 Peter Zijlstra <peterz@infradead.org>:
> On Wed, Nov 25, 2015 at 03:43:54PM +0100, Peter Zijlstra wrote:
>> On Mon, Sep 21, 2015 at 04:01:41PM -0400, Tejun Heo wrote:
>> > So, the only way the patch could have caused the above is if someone
>> > who isn't the task itself is writing to the bitfields while the task
>> > is running.  Looking through the fields, ->sched_reset_on_fork seems a
>> > bit suspicious.  __sched_setscheduler() looks like it can modify the
>> > bit while the target task is running.  Peter, am I misreading the
>> > code?
>>
>> Nope, that's quite possible. Looks like we need to break up those
>> bitfields a bit. All the scheduler ones should be serialized by
>> scheduler locks, but the others are fair game.
>
> Maybe something like so; but my brain is a complete mess today.
>
> ---
>  include/linux/sched.h | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f425aac63317..b474e0f05327 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1455,14 +1455,15 @@ struct task_struct {
>         /* Used for emulating ABI behavior of previous Linux versions */
>         unsigned int personality;
>
> -       unsigned in_execve:1;   /* Tell the LSMs that the process is doing an
> -                                * execve */
> -       unsigned in_iowait:1;
> -
> -       /* Revert to default priority/policy when forking */
> +       /* scheduler bits, serialized by scheduler locks */
>         unsigned sched_reset_on_fork:1;
>         unsigned sched_contributes_to_load:1;
>         unsigned sched_migrated:1;
> +       unsigned __padding_sched:29;

AFAIK the order of bit fields is implementation defined, so GCC could
sort all these bits as it wants.
You could use unnamed zero-widht bit-field to force padding:

         unsigned :0; //force aligment to the next boundary.

> +
> +       /* unserialized, strictly 'current' */
> +       unsigned in_execve:1; /* bit to tell LSMs we're in execve */
> +       unsigned in_iowait:1;
>  #ifdef CONFIG_MEMCG
>         unsigned memcg_may_oom:1;
>  #endif
>

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