On Mon, Apr 03, 2017 at 02:18:44PM -0400, Jeff Layton wrote: > On Mon, 2017-04-03 at 11:09 -0700, Jeremy Allison wrote: > > On Mon, Apr 03, 2017 at 01:47:37PM -0400, Jeff Layton wrote: > > > On Mon, 2017-04-03 at 07:32 -0700, Matthew Wilcox wrote: > > > > On Mon, Apr 03, 2017 at 06:28:38AM -0400, Jeff Layton wrote: > > > > > On Mon, 2017-04-03 at 14:25 +1000, NeilBrown wrote: > > > > > > Also I think that EIO should always over-ride ENOSPC as the possible > > > > > > responses are different. That probably means you need a separate seq > > > > > > number for each, which isn't ideal. > > > > > > > > > > > > > > > > I'm not quite convinced that it's really useful to do anything but > > > > > report the latest error. > > > > > > > > > > But...if we did need to prefer one over another, could we get away with > > > > > always reporting -EIO once that error occurs? If so, then we'd still > > > > > just need a single sequence counter. > > > > > > > > I wonder whether it's even worth supporting both EIO and ENOSPC for a > > > > writeback problem. If I understand correctly, at the time of write(), > > > > filesystems check to see if they have enough blocks to satisfy the > > > > request, so ENOSPC only comes up in the writeback context for thinly > > > > provisioned devices. > > > > > > > > Programs have basically no use for the distinction. In either case, > > > > the situation is the same. The written data is safely in RAM and cannot > > > > be written to the storage. If one were to make superhuman efforts, > > > > one could mmap the file and write() it to a different device, but that > > > > is incredibly rare. For most programs, the response is to just die and > > > > let the human deal with the corrupted file. > > > > > > > > From a sysadmin point of view, of course the situation is different, > > > > and the remedy is different, but they should be getting that information > > > > through a different mechanism than monitoring the errno from every > > > > system call. > > > > > > > > If we do want to continue to support both EIO and ENOSPC from writeback, > > > > then let's have EIO override ENOSPC as an error. ie if an ENOSPC comes > > > > in after an EIO is set, it only bumps the counter and applications will > > > > see EIO, not ENOSPC on fresh calls to fsync(). > > > > > > > > > No, ENOSPC on writeback can certainly happen with network filesystems. > > > NFS and CIFS have no way to reserve space. You wouldn't want to have to > > > do an extra RPC on every buffered write. :) > > > > CIFS has a way to reserve space. Look into "allocation size" on create. > > That won't help here as it's done on open(). > > The problem here is that we might create a file (and not preallocate > anything), then write a bunch of stuff to the cache under an oplock. > Then when we go to write back, we get the CIFS equivalent of -ENOSPC. > > What local filesystems do (AIUI) is preallocate so that you can catch > an ENOSPC condition earlier, when you're dirtying new pages in the > cache. That's pretty much impossible to do on a network filesystem > though. There's also SMB_SET_FILE_ALLOCATION_INFO which can be done over SMB1/2/3 on an open file handle.
