linux-kernel - Re: [PATCH 2/2] fs: wire up .truncate

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Date:	Wed, 23 Nov 2011 11:16:47 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Hellwig <hch@....de>
cc:	Cong Wang <amwang@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Al Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] fs: wire up .truncate_range and .fallocate

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);
}
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