Re: [RFP-V2 0/3] Make mmu_notifier_invalidate_range_start able to sleep.

[Robin Holt <holt@sgi.com>]

On Tue, Feb 02, 2010 at 05:52:24PM +0100, Andrea Arcangeli wrote:
> On Tue, Feb 02, 2010 at 10:39:30AM -0600, Robin Holt wrote:
> > On Tue, Feb 02, 2010 at 05:01:46PM +0100, Andrea Arcangeli wrote:
> > > On Tue, Feb 02, 2010 at 09:21:42AM -0600, Robin Holt wrote:
> > > > unmap_mapping_range_vma would then unlock the i_mmap_lock, call
> > > > _inv_range_start(atomic==0) which would clear all the remote page tables
> > > > and TLBs.  It would then reaquire the i_mmap_lock and retry.
> > > 
> > > I guess you missed why we hold the i_mmap_lock there... it's not like
> > > gratuitous locking complication there's an actual reason why it's
> > > taken, and it's to avoid the vma to be freed from under you:
> > 
> > Oversight on my part.  Sorry.
> > 
> > Will this work?
> 
> No, it still corrupts memory as before. You need to re-run find_vma
> under mmap_sem. Then it could work...

But we don't use the vma for anything.  The _invalidate_range_start/end
is using the mm.  XPMEM and GRU don't use the vma.  Does KVM?  Since it
isn't passed in, I would expect that anybody trying to use the vma is
going to have to do a find_vma themselves.  Did I miss something?

> Also it's wrong to pin mm_users, you only need mm_count here, as you
> only need to run find_vma, you don't need to prevent exit to free the
> pages indefinitely while you're blocked.

Is this better?

static int unmap_mapping_range_vma(struct vm_area_struct *vma,
...
	if (need_unlocked_invalidate) {
		mm = vma->vm_mm;
		atomic_inc(&mm->mm_count);
	}
	spin_unlock(details->i_mmap_lock);
	if (need_unlocked_invalidate) {
		/*
		 * zap_page_range failed to make any progress because the
		 * mmu_notifier_invalidate_range_start was called atomically
		 * while the callee needed to sleep.  In that event, we
		 * make the callout while the i_mmap_lock is released.
		 */
		mmu_notifier_invalidate_range_start(mm, start_addr, end_addr, 0);
		mmu_notifier_invalidate_range_end(mm, start_addr, end_addr);
		mmdrop(mm);
	}

Robin

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