On Thu, Mar 31, 2016 at 02:08:21PM -0400, J. Bruce Fields wrote: > On Thu, Mar 31, 2016 at 10:18:50PM +1100, Dave Chinner wrote: > > On Thu, Mar 31, 2016 at 12:54:40AM -0700, Christoph Hellwig wrote: > > > On Thu, Mar 31, 2016 at 12:18:13PM +1100, Dave Chinner wrote: > > > > On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote: > > > > > Or is it ok that fallocate could block, potentially for a long time as > > > > > we stream cows through the page cache (or however unshare works > > > > > internally)? Those same programs might not be expecting fallocate to > > > > > take a long time. > > > > > > > > Yes, it's perfectly fine for fallocate to block for long periods of > > > > time. See what gfs2 does during preallocation of blocks - it ends up > > > > calling sb_issue_zerout() because it doesn't have unwritten > > > > extents, and hence can block for long periods of time.... > > > > > > gfs2 fallocate is an implementation that will cause all but the most > > > trivial users real pain. Even the initial XFS implementation just > > > marking the transactions synchronous made it unusable for all kinds > > > of applications, and this is much worse. E.g. a NFS ALLOCATE operation > > > to gfs2 will probab;ly hand your connection for extended periods of > > > time. > > > > > > If we need to support something like what gfs2 does we should have a > > > separate flag for it. > > > > Using fallocate() for preallocation was always intended to > > be a faster, more efficient method allocating zeroed space > > than having userspace write blocks of data. Faster, more efficient > > does not mean instantaneous, and gfs2 using sb_issue_zerout() means > > that if the hardware has zeroing offloads (deterministic trim, write > > same, etc) it will use them, and that will be much faster than > > writing zeros from userspace. > > > > IMO, what gfs2 is definitely within the intended usage of > > fallocate() for accelerating the preallocation of blocks. > > > > Yes, it may not be optimal for things like NFS servers which haven't > > considered that a fallocate based offload operation might take some > > time to execute, but that's not a problem with fallocate. i.e. > > that's a problem with the nfs server ALLOCATE implementation not > > being prepared to return NFSERR_JUKEBOX to prevent client side hangs > > and timeouts while the operation is run.... > > That's an interesting idea, but I don't think it's really legal. I take > JUKEBOX to mean "sorry, I'm failing this operation for now, try again > later and it might succeed", not "OK, I'm working on it, try again and > you may find out I've done it". > > So if the client gets a JUKEBOX error but the server goes ahead and does > the operation anyway, that'd be unexpected. > > I suppose it's comparable to the case where a slow fallocate is > interrupted--would it be legal to return EINTR in that case and leave > the application to sort out whether some part of the allocation had > already happened? <shrug> The unshare component to XFS fallocate does this if something sends a fatal signal to the process. There's a difference between shooting down a process in the middle of fallocate and fallocate returning EINTR out of the blue, though... ...the manpage for fallocate says that "EINTR == a signal was caught". > Would it be legal to continue the fallocate under the covers even > after returning EINTR? It doesn't do that, however. --D > But anyway my first inclination is to say that the NFS FALLOCATE > protocol just wasn't designed to handle long-running fallocates, and if > we really need that then we need to give it a way to either report > partial results or to report results asynchronously. > > --b. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
