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Hole punching and mmap races

To: linux-fsdevel@xxxxxxxxxxxxxxx
Subject: Hole punching and mmap races
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 16 May 2012 00:48:05 +0200
Cc: xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, Hugh Dickins <hughd@xxxxxxxxxx>, linux-mm@xxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)

  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)
addr = mmap(file);
addr[0] = 1
  ^^ writeably fault a page
                                  remove file blocks

                                                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?

Jan Kara <jack@xxxxxxx>

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