linux-kernel - Re: [patch 3/6] mm: fix fault vs invalidate race for linear mappings

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Message-ID: <20070307072545.GC15877@wotan.suse.de>
Date:	Wed, 7 Mar 2007 08:25:46 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [patch 3/6] mm: fix fault vs invalidate race for linear mappings

On Tue, Mar 06, 2007 at 11:08:41PM -0800, Andrew Morton wrote:
> On Wed, 7 Mar 2007 07:57:27 +0100 Nick Piggin <npiggin@...e.de> wrote:
> 
> > > 
> > > Why was truncate_inode_pages_range() altered to unmap the page if it got
> > > mapped again?
> > > 
> > > Oh.  Because the unmap_mapping_range() call got removed from vmtruncate(). 
> > > Why?  (Please send suitable updates to the changelog).
> > 
> > We have to ensure it is unmapped, and be prepared to unmap it while under
> > the page lock.
> 
> But vmtruncate() dropped i_size, so nobody will map this page into
> pagetables from then on.

But there could be a fault in progress... the only way to know is
locking the page.

> > > I guess truncate of a mmapped area isn't sufficiently common to worry about
> > > the inefficiency of this change.
> > 
> > Yeah, and it should be more efficient for files that aren't mmapped,
> > because we don't have to take i_mmap_lock for them.
> > 
> > > Lots of memory barriers got removed in memory.c, unchangeloggedly.
> > 
> > Yeah they were all for the lockless truncate_count checks. Now that
> > we use the page lock, we don't need barriers.
> > 
> > > Gratuitous renaming of locals in do_no_page() makes the change hard to
> > > review.  Should have been a separate patch.
> > > 
> > > In fact, the patch would have been heaps clearer if that renaming had been
> > > a separate patch.
> > 
> > Shall I?
> 
> If you don't have anything better to do, yes please ;)

OK.
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