Re: 2.6.22 -mm merge plans -- vm bugfixes

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Thu, 10 May 2007, Nick Piggin wrote:
>
> > > 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?
> >
> > The filesystem still doesn't, but if i_size is updated after the page
> > is returned, we can have a problem that was previously taken care of
> > with the truncate_count but now isn't.
>
>
> But... I thought the page lock was now taking care of that in your
> scheme?  truncate_inode_pages has to wait for the page lock, then
> it finds the page is mapped and... ahh, it finds the copiee page
> is not mapped, so doesn't do its own little unmap_mapping_range,
> and the copied page squeaks through.  Drat.
>
> I really think the truncate_count solution worked better, for
> truncation anyway.  There may be persuasive reasons you need the
> page lock for invalidation: I gave up on trying to understand the
> required behaviour(s) for invalidation.
>
> So, bring back (the original use of, not my tree marker use of)
> truncate_count?  Hmm, you probably don't want to do that, because
> there was some pleasure in removing the strange barriers associated
> with it.
>
> A second unmap_mapping_range is just one line of code - but it sure
> feels like a defeat to me, calling the whole exercise into question.
> (But then, you'd be right to say my perfectionism made it impossible
> for me to come up with any solution to the invalidation issues.)

Well we could bring back the truncate_count, but I think that sucks
because that's moving work into the page fault handler in order to
avoid a bit of work when truncating mapped files.


> > > But that's a change we could have made at
> > > any time if we'd bothered, it's not really the issue here.
> >
> > I don't see how you could, because you need to increment truncate_count.
>
>
> Though indeed we did so, I don't see that we needed to increment
> truncate_count in that case (nobody could be coming through
> do_no_page on that file, when there are no mappings of it).

Of course :P

--
SUSE Labs, Novell Inc.
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