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@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
