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Date: Wed, 23 Nov 2011 11:16:47 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Christoph Hellwig <hch@lst.de>
cc: Cong Wang <amwang@redhat.com>, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, Al Viro <viro@zeniv.linux.org.uk>,
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        Johannes Weiner <hannes@cmpxchg.org>, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 2/2] fs: wire up .truncate_range and .fallocate
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On Wed, 23 Nov 2011, Christoph Hellwig wrote:
> 
> It also seems like all fallocate implementaions for far got away
> without the unmap_mapping_range, so either people didn't test them
> hard enough, or tmpfs doesn't need it either.  I fear the former
> is true.

They're saved by the funny little one-by-one unmap_mapping_range()
fallback in truncate_inode_page().  It's inefficient (in those rare
cases when someone is punching a hole somewhere that's mapped) and
we ought to do better, but we don't have an actual bug there.

Hugh

int truncate_inode_page(struct address_space *mapping, struct page *page)
{
	if (page_mapped(page)) {
		unmap_mapping_range(mapping,
				   (loff_t)page->index << PAGE_CACHE_SHIFT,
				   PAGE_CACHE_SIZE, 0);
	}
	return truncate_complete_page(mapping, page);
}