Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and get_user_pages_unlocked

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Oct 09, 2014 at 12:50:37PM +0200, Peter Zijlstra wrote:
> 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 second (and third) caller pass notify_drop=false, so the
notify_drop parameter is always a noop for them. They certainly could
get away without it.

> the exact same thing afterwards anyway.

It makes a difference only to the first caller, if it wasn't for the
first caller notify_drop could be dropped. The first caller does this:

	return __get_user_pages_locked(tsk, mm, start, nr_pages, write, force,
				       pages, NULL, locked, true, FOLL_TOUCH);
				       	      	            ^ notify_drop = true

Without "notify_drop=true" the first caller could make its own
respective caller think the lock has never been dropped, just because
it is locked by the time get_user_pages_locked returned. But the
caller must be made aware that the lock has been dropped during the
call and in turn any "vma" it got before inside the mmap_sem critical
section is now stale. That's all notify_drop achieves.

Thanks,
Andrea

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