linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

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Date:	Wed, 9 May 2007 15:28:31 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

On Wed, 9 May 2007, Nick Piggin wrote:
> Hugh Dickins wrote:
> > On Wed, 2 May 2007, Nick Piggin wrote:
> 
> > > >But I'm pretty sure (to use your words!) regular truncate was not racy
> > > >before: I believe Andrea's sequence count was handling that case fine,
> > > >without a second unmap_mapping_range.
> > >
> > >OK, I think you're right. I _think_ it should also be OK with the
> > >lock_page version as well: we should not be able to have any pages
> > >after the first unmap_mapping_range call, because of the i_size
> > >write. So if we have no pages, there is nothing to 'cow' from.
> > 
> > I'd be delighted if you can remove those later unmap_mapping_ranges.
> > As I recall, the important thing for the copy pages is to be holding
> > the page lock (or whatever other serialization) on the copied page
> > still while the copy page is inserted into pagetable: that looks
> > to be so in your __do_fault.
> 
> Hmm, on second thoughts, I think I was right the first time, and do
> need the unmap after the pages are truncated. With the lock_page code,
> after the first unmap, we can get new ptes mapping pages, and
> subsequently they can be COWed and then the original pte zapped before
> the truncate loop checks it.

The filesystem (or page cache) allows pages beyond i_size to come
in there?  That wasn't a problem before, was it?  But now it is?

> 
> However, I wonder if we can't test mapping_mapped before the spinlock,
> which would make most truncates cheaper?

Slightly cheaper, yes, though I doubt it'd be much in comparison with
actually doing any work in unmap_mapping_range or truncate_inode_pages.
Suspect you'd need a barrier of some kind between the i_size_write and
the mapping_mapped test?  But that's a change we could have made at
any time if we'd bothered, it's not really the issue here.

Hugh
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