linux-ext4 - Hole punching and mmap races

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Message-ID: <20120515224805.GA25577@quack.suse.cz>
Date:	Wed, 16 May 2012 00:48:05 +0200
From:	Jan Kara <jack@...e.cz>
To:	linux-fsdevel@...r.kernel.org
Cc:	xfs@....sgi.com, linux-ext4@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org
Subject: Hole punching and mmap races

  Hello,

  Hugh pointed me to ext4 hole punching code which is clearly missing some
locking. But looking at the code more deeply I realized I don't see
anything preventing the following race in XFS or ext4:

TASK1				TASK2
				punch_hole(file, 0, 4096)
				  filemap_write_and_wait()
				  truncate_pagecache_range()
addr = mmap(file);
addr[0] = 1
  ^^ writeably fault a page
				  remove file blocks

						FLUSHER
						write out file
						  ^^ interesting things can
happen because we expect blocks under the first page to be allocated /
reserved but they are not...

I'm pretty sure ext4 has this problem, I'm not completely sure whether
XFS has something to protect against such race but I don't see anything.

It's not easy to protect against these races. For truncate, i_size protects
us against similar races but for hole punching we don't have any such
mechanism. One way to avoid the race would be to hold mmap_sem while we are
invalidating the page cache and punching hole but that sounds a bit ugly.
Alternatively we could just have some special lock (rwsem?) held during
page_mkwrite() (for reading) and during whole hole punching (for writing)
to serialize these two operations.

Another alternative, which doesn't really look more appealing, is to go
page-by-page and always free corresponding blocks under page lock.

Any other ideas or thoughts?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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