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

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Date:	Wed, 1 Oct 2014 08:51:59 -0700
From:	Peter Feiner <pfeiner@...gle.com>
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>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	"\\\"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)
> +{
> +	int flags = FOLL_TOUCH;
> +	long ret, pages_done;
> +	bool lock_dropped;
> +
> +	if (locked) {
> +		/* if VM_FAULT_RETRY can be returned, vmas become invalid */
> +		BUG_ON(vmas);
> +		/* check caller initialized locked */
> +		BUG_ON(*locked != 1);
> +	}
> +
> +	if (pages)
> +		flags |= FOLL_GET;
> +	if (write)
> +		flags |= FOLL_WRITE;
> +	if (force)
> +		flags |= FOLL_FORCE;
> +
> +	pages_done = 0;
> +	lock_dropped = false;
> +	for (;;) {
> +		ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
> +				       vmas, locked);
> +		if (!locked)
> +			/* VM_FAULT_RETRY couldn't trigger, bypass */
> +			return ret;
> +
> +		/* VM_FAULT_RETRY cannot return errors */
> +		if (!*locked) {
> +			BUG_ON(ret < 0);
> +			BUG_ON(nr_pages == 1 && ret);

If I understand correctly, this second BUG_ON is asserting that when
__get_user_pages is asked for a single page and it is successfully gets the
page, then it shouldn't have dropped the mmap_sem. If that's the case, then
you could generalize this assertion to

			BUG_ON(nr_pages == ret);

Otherwise, looks good!

Peter
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