Hole punching and mmap races

Jan Kara
2012-05-15T22:48:27Z

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?