linux-kernel - Re: [PATCH -mmotm-2010-01-06-14-34] Fix fault count of task in GUP

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Message-ID: <alpine.LSU.2.00.1001112320490.7893@sister.anvils>
Date:	Mon, 11 Jan 2010 23:29:37 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Minchan Kim <minchan.kim@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mmotm-2010-01-06-14-34] Fix fault count of task in GUP

On Mon, 11 Jan 2010, Minchan Kim wrote:
> 
> get_user_pages calls handle_mm_fault to pin the arguemented
> task's page. handle_mm_fault cause major or minor fault and
> get_user_pages counts it into task which is passed by argument.
> 
> But the fault happens in current task's context.
> So we have to count it not argumented task's context but current
> task's one.

Have to?

current simulates a fault into tsk's address space.
It is not a fault into current's address space.

I can see that this could be argued either way, or even
that such a "fault" should not be counted at all; but I do not
see a reason to change the way we have been counting it for years.

Sorry, but NAK (to this and to the v2) -
unless you have a stronger argument.

Hugh

> 
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> CC: Nick Piggin <npiggin@...e.de>
> CC: Hugh Dickins <hugh.dickins@...cali.co.uk>
> ---
>  mm/memory.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 521abf6..2513581 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1486,9 +1486,9 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  					BUG();
>  				}
>  				if (ret & VM_FAULT_MAJOR)
> -					tsk->maj_flt++;
> +					current->maj_flt++;
>  				else
> -					tsk->min_flt++;
> +					current->min_flt++;
>  
>  				/*
>  				 * The VM_FAULT_WRITE bit tells us that
> -- 
> 1.5.6.3
> 
> 
> -- 
> Kind regards,
> Minchan Kim
--
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