linux-kernel - Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and get_user_pages

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Message-ID: <20141009105037.GM4750@worktop.programming.kicks-ass.net>
Date:	Thu, 9 Oct 2014 12:50:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andres Lagar-Cavilla <andreslc@...gle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"\\\"Dr. David Alan Gilbert\\\"" <dgilbert@...hat.com>
Subject: Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and
 get_user_pages_unlocked

On Wed, Oct 01, 2014 at 10:56:35AM +0200, Andrea Arcangeli wrote:

> +static inline long __get_user_pages_locked(struct task_struct *tsk,
> +					   struct mm_struct *mm,
> +					   unsigned long start,
> +					   unsigned long nr_pages,
> +					   int write, int force,
> +					   struct page **pages,
> +					   struct vm_area_struct **vmas,
> +					   int *locked,
> +					   bool notify_drop)
> +{

> +	if (notify_drop && lock_dropped && *locked) {
> +		/*
> +		 * We must let the caller know we temporarily dropped the lock
> +		 * and so the critical section protected by it was lost.
> +		 */
> +		up_read(&mm->mmap_sem);
> +		*locked = 0;
> +	}
> +	return pages_done;
> +}

> +long get_user_pages_locked(struct task_struct *tsk, struct mm_struct *mm,
> +			   unsigned long start, unsigned long nr_pages,
> +			   int write, int force, struct page **pages,
> +			   int *locked)
> +{
> +	return __get_user_pages_locked(tsk, mm, start, nr_pages, write, force,
> +				       pages, NULL, locked, true);
> +}

> +long get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
> +			     unsigned long start, unsigned long nr_pages,
> +			     int write, int force, struct page **pages)
> +{
> +	long ret;
> +	int locked = 1;
> +	down_read(&mm->mmap_sem);
> +	ret = __get_user_pages_locked(tsk, mm, start, nr_pages, write, force,
> +				      pages, NULL, &locked, false);
> +	if (locked)
> +		up_read(&mm->mmap_sem);
> +	return ret;
> +}

>  long get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  		unsigned long start, unsigned long nr_pages, int write,
>  		int force, struct page **pages, struct vm_area_struct **vmas)
>  {
> +	return __get_user_pages_locked(tsk, mm, start, nr_pages, write, force,
> +				       pages, vmas, NULL, false);
>  }

I'm wondering about that notify_drop parameter, what's the added
benefit? If you look at these 3 callers we can do away with it, since in
the second called where we have locked but !notify_drop we seem to do
the exact same thing afterwards anyway.
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