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Date: Mon, 16 Apr 2012 14:44:22 +0100
From: Mel Gorman <mgorman@suse.de>
To: Fred Isaman <iisaman@netapp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Linux-MM <linux-mm@kvack.org>,
        Linux-Netdev <netdev@vger.kernel.org>,
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        Mike Christie <michaelc@cs.wisc.edu>,
        Eric B Munson <emunson@mgebm.net>
Subject: Re: [PATCH 08/11] nfs: disable data cache revalidation for swapfiles
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On Mon, Apr 16, 2012 at 09:10:04AM -0400, Fred Isaman wrote:
> > <SNIP>
> > -static struct nfs_page *nfs_page_find_request_locked(struct page *page)
> > +static struct nfs_page *
> > +nfs_page_find_request_locked(struct nfs_inode *nfsi, struct page *page)
> > ?{
> > ? ? ? ?struct nfs_page *req = NULL;
> >
> > - ? ? ? if (PagePrivate(page)) {
> > + ? ? ? if (PagePrivate(page))
> > ? ? ? ? ? ? ? ?req = (struct nfs_page *)page_private(page);
> > - ? ? ? ? ? ? ? if (req != NULL)
> > - ? ? ? ? ? ? ? ? ? ? ? kref_get(&req->wb_kref);
> > + ? ? ? else if (unlikely(PageSwapCache(page))) {
> > + ? ? ? ? ? ? ? struct nfs_page *freq, *t;
> > +
> > + ? ? ? ? ? ? ? /* Linearly search the commit list for the correct req */
> > + ? ? ? ? ? ? ? list_for_each_entry_safe(freq, t, &nfsi->commit_list, wb_list) {
> > + ? ? ? ? ? ? ? ? ? ? ? if (freq->wb_page == page) {
> > + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? req = freq;
> > + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? break;
> > + ? ? ? ? ? ? ? ? ? ? ? }
> > + ? ? ? ? ? ? ? }
> > +
> > + ? ? ? ? ? ? ? BUG_ON(req == NULL);
> 
> I suspect I am missing something, but why is it guaranteed that the
> req is on the commit list?
> 

It's a fair question and a statement about what I expected to happen.
The commit list replaces the nfs_page_tree radix tree that used to exist
and my understanding was that the req would exist in the radix tree until
the swap IO was completed. I expected it to be the same for the commit
list and the BUG_ON was based on that expectation. Are there cases where
the req would not be found?

Thanks.

-- 
Mel Gorman
SUSE Labs