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nfsd_splice_actor() has a clause in its loop that chops up a compound page
into individual pages such that if the same page is seen twice in a row, it
is discarded the second time.  This is a problem with the advent of
shmem_splice_read() as that inserts zero_pages into the pipe in lieu of
pages that aren't present in the pagecache.

Fix this by assuming that the last page is being extended only if the
currently stored length + starting offset is not currently on a page
boundary.

This can be tested by NFS-exporting a tmpfs filesystem on the test machine
and truncating it to more than a page in size (eg. truncate -s 8192) and
then reading it by NFS.  The first page will be all zeros, but thereafter
garbage will be read.

Note: I wonder if we can ever get a situation now where we get a splice
that gives us contiguous parts of a page in separate actor calls.  As NFSD
can only be splicing from a file (I think), there are only three sources of
the page: copy_splice_read(), shmem_splice_read() and file_splice_read().
The first allocates pages for the data it reads, so the problem cannot
occur; the second should never see a partial page; and the third waits for
each page to become available before we're allowed to read from it.

Fixes: bd194b18

 ("shmem: Implement splice-read")
Reported-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
cc: Hugh Dickins <hughd@google.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>