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Date: Thu, 05 Sep 2002 14:12:23 -0700
From: Andrew Morton <akpm@zip.com.au>
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To: trond.myklebust@fys.uio.no
CC: Chuck Lever <cel@citi.umich.edu>,
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Subject: Re: invalidate_inode_pages in 2.5.32/3
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Trond Myklebust wrote:
> 
> >>>>> " " == Andrew Morton <akpm@zip.com.au> writes:
> 
>      > Probably, it worked OK with the global locking because nobody
>      > was taking a temp ref against those pages.
> 
> But we still have global locking in that readdir by means of the
> BKL. There should be no nasty races...
> 
>      > Please tell me exactly what semantics NFS needs in there.  Does
>      > truncate_inode_pages() do the wrong thing?
> 
> Definitely. In most cases we *cannot* wait on I/O completion because
> nfs_zap_caches() can be called directly from the 'rpciod' process by
> means of a callback. Since 'rpciod' is responsible for completing all
> asynchronous I/O, then truncate_inode_pages() would deadlock.

But if there are such pages, invalidate_inode_pages() would not
have removed them anyway?

> As I said: I don't believe the problem here has anything to do with
> invalidate_inode_pages vs. truncate_inode_pages:
>   - Pages should only be locked if they are actually being read from
>     the server.
>   - They should only be refcounted and/or cleared while the BKL is
>     held...
> There is no reason why code which worked fine under 2.2.x and 2.4.x
> shouldn't work under 2.5.x.

Trond, there are very good reasons why it broke.  Those pages are
visible to the whole world via global data structures - both the 
page LRUs and via the superblocks->inodes walk.  Those things exist
for legitimate purposes, and it is legitimate for async threads
of control to take a reference on those pages while playing with them.

It just "happened to work" in earlier kernels.

I suspect we can just remove the page_count() test from invalidate
and that will fix everything up.  That will give stronger invalidate
and anything which doesn't like that is probably buggy wrt truncate anyway.

Could you test that?
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