On Thu, 3 Nov 2022, hev wrote: > On Wed, Nov 2, 2022 at 10:41 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Nov 01, 2022 at 11:22:48AM +0800, Rui Wang wrote: > > > This patch fixes data loss caused by the fallocate system > > > call interrupted by a signal. > > > > > > Bug: https://lore.kernel.org/linux-mm/33b85d82.7764.1842e9ab207.Coremail.chenguoqic@xxxxxxx/ > > > Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios") > > > > How does that commit introduce this bug? > > In the test case[1], we created a file that contains non-zero data > from offset 0 to A-1. and a process try to expand this file by > fallocate(fd, 0, 0, B), B > A. > Concurrently, another process try to interrupt this fallocate syscall > by a signal. I think the expected results are: > > 1. The file is not expanded and file size is A, and the data from > offset 0 to A-1 is not changed. > 2. The file is expanded and the data from offset 0 to A-1 is not > changed, and from A to B-1 contains zeros. > > Now, the unexpected result is that the file is not expanded and the > data that from offset 0 to A-1 is changed by > truncate_inode_partial_folio that called > from shmem_undo_range with unfalloc = true. > > This issue is only reproduced when file on tmpfs, and begin from this > commit: b9a8a4195c7d ("truncate,shmem: Handle truncates that split > large folios") Like Matthew, I was sceptical at first. But I currently think that you have discovered something important, and that your patch is the correct fix to it; but I'm still rather confused, and want to do some more thinking and testing: this mail is mainly to signal to Matthew that I'm on it, so he doesn't have to rush to look at it when he's back. I was able to reproduce it with the test case, once I multiplied both of the usleep intervals by 10 - before that, it was too difficult for it to complete a fallocate: guess the timing is different on my x86 box. Hugh
