linux-kernel - Re: [RFC] perf: mmap2 not covering VM

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Date:	Mon, 7 Oct 2013 23:04:38 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] perf: mmap2 not covering VM_CLONE regions

Peter,


On Thu, Oct 3, 2013 at 11:03 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Oct 03, 2013 at 10:55:28AM +0200, Stephane Eranian wrote:
>> I don't know the MM code but I assume that that vm_mm struct is
>> allocated dynamically
>> and maybe you already grabbing a lock while doing this. Could we
>> leverage that lock
>> to increment a global generation number?
>
> Sure; something like so.. I just don't like global state nor adding to
> mm_struct for just this.
>
> ---
>  include/linux/mm_types.h | 1 +
>  kernel/fork.c            | 6 ++++++
>  2 files changed, 7 insertions(+)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index d9851eeb6e1d..3877b1e72a5b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -436,6 +436,7 @@ struct mm_struct {
>         int first_nid;
>  #endif
>         struct uprobes_state uprobes_state;
> +       u64 mm_id;
>  };
>
>  /* first nid will either be a valid NID or one of these values */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 086fe73ad6bd..b315f6227629 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -523,6 +523,8 @@ static void mm_init_aio(struct mm_struct *mm)
>  #endif
>  }
>
> +static u64 global_mm_id;
> +
>  static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
>  {
>         atomic_set(&mm->mm_users, 1);
> @@ -537,6 +539,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
>         spin_lock_init(&mm->page_table_lock);
>         mm_init_aio(mm);
>         mm_init_owner(mm, p);
> +       mm->mm_id = 0;
>
>         if (likely(!mm_alloc_pgd(mm))) {
>                 mm->def_flags = 0;
> @@ -1422,6 +1425,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>          */
>         write_lock_irq(&tasklist_lock);
>
> +       if (p->mm && !p->mm->mm_id)
> +               p->mm->mm_id = ++global_mm_id;
> +
>         /* CLONE_PARENT re-uses the old parent */
>         if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
>                 p->real_parent = current->real_parent;

Ok, so I tried this. It does not cover shmat() cases unfortunately.
We need that same ++global_mm_id somewhere on that code path.
Haven't looked at it in details just yet.
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