On Mon, Feb 11, 2019 at 12:49 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Mon, Feb 11, 2019 at 11:58:47AM -0800, Dan Williams wrote: > > On Mon, Feb 11, 2019 at 10:40 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > On Mon, Feb 11, 2019 at 11:26:49AM -0700, Jason Gunthorpe wrote: > > > > On Mon, Feb 11, 2019 at 10:19:22AM -0800, Ira Weiny wrote: > > > > > What if user space then writes to the end of the file with a regular write? > > > > > Does that write end up at the point they truncated to or off the end of the > > > > > mmaped area (old length)? > > > > > > > > IIRC it depends how the user does the write.. > > > > > > > > pwrite() with a given offset will write to that offset, re-extending > > > > the file if needed > > > > > > > > A file opened with O_APPEND and a write done with write() should > > > > append to the new end > > > > > > > > A normal file with a normal write should write to the FD's current > > > > seek pointer. > > > > > > > > I'm not sure what happens if you write via mmap/msync. > > > > > > > > RDMA is similar to pwrite() and mmap. > > > > > > A pertinent point that you didn't mention is that ftruncate() does not change > > > the file offset. So there's no user-visible change in behaviour. > > > > ...but there is. The blocks you thought you freed, especially if the > > system was under -ENOSPC pressure, won't actually be free after the > > successful ftruncate(). > > They won't be free after something dirties the existing mmap either. > > Blocks also won't be free if you unlink a file that is currently still > open. > > This isn't really new behavior for a FS. An mmap write after a fault due to a hole punch is free to trigger SIGBUS if the subsequent page allocation fails. So no, I don't see them as the same unless you're allowing for the holder of the MR to receive a re-fault failure.
