On Wed, Jun 23, 2021 at 08:06:39AM +1000, Dave Chinner wrote: > On Mon, Jun 21, 2021 at 01:55:03PM -0700, Omar Sandoval wrote: > > On Mon, Jun 21, 2021 at 01:46:04PM -0700, Omar Sandoval wrote: > > > On Mon, Jun 21, 2021 at 12:33:17PM -0700, Linus Torvalds wrote: > > > > On Mon, Jun 21, 2021 at 11:46 AM Omar Sandoval <osandov@xxxxxxxxxxx> wrote: > > > > > > > > > > How do we get the userspace size with the encoded_iov.size approach? > > > > > We'd have to read the size from the iov_iter before writing to the rest > > > > > of the iov_iter. Is it okay to mix the iov_iter as a source and > > > > > destination like this? From what I can tell, it's not intended to be > > > > > used like this. > > > > > > > > I guess it could work that way, but yes, it's ugly as hell. And I > > > > really don't want a readv() system call - that should write to the > > > > result buffer - to first have to read from it. > > > > > > > > So I think the original "just make it be the first iov entry" is the > > > > better approach, even if Al hates it. > > > > > > > > Although I still get the feeling that using an ioctl is the *really* > > > > correct way to go. That was my first reaction to the series > > > > originally, and I still don't see why we'd have encoded data in a > > > > regular read/write path. > > > > > > > > What was the argument against ioctl's, again? > > > > > > The suggestion came from Dave Chinner here: > > > https://lore.kernel.org/linux-fsdevel/20190905021012.GL7777@xxxxxxxxxxxxxxxxxxx/ > > > > > > His objection to an ioctl was two-fold: > > > > > > 1. This interfaces looks really similar to normal read/write, so we > > > should try to use the normal read/write interface for it. Perhaps > > > this trouble with iov_iter has refuted that. > > > 2. The last time we had Btrfs-specific ioctls that eventually became > > > generic (FIDEDUPERANGE and FICLONE{,RANGE}), the generalization was > > > painful. Part of the problem with clone/dedupe was that the Btrfs > > > ioctls were underspecified. I think I've done a better job of > > > documenting all of the semantics and corner cases for the encoded I/O > > > interface (and if not, I can address this). The other part of the > > > problem is that there were various sanity checks in the normal > > > read/write paths that were missed or drifted out of sync in the > > > ioctls. That requires some vigilance going forward. Maybe starting > > > this off as a generic (not Btrfs-specific) ioctl right off the bat > > > will help. > > > > > > If we do go the ioctl route, then we also have to decide how much of > > > preadv2/pwritev2 it should emulate. Should it use the fd offset, or > > > should that be an ioctl argument? Some of the RWF_ flags would be useful > > > for encoded I/O, too (RWF_DSYNC, RWF_SYNC, RWF_APPEND), should it > > > support those? These bring us back to Dave's first point. > > > > Oops, I dropped Dave from the Cc list at some point. Adding him back > > now. > > Fair summary. The only other thing that I'd add is this is an IO > interface that requires issuing physical IO. So if someone wants > high throughput for encoded IO, we really need AIO and/or io_uring > support, and we get that for free if we use readv2/writev2 > interfaces. > > Yes, it could be an ioctl() interface, but I think that this sort of > functionality is exactly what extensible syscalls like > preadv2/pwritev2 should be used for. It's a slight variant on normal > IO, and that's exactly what the RWF_* flags are intended to be used > for - allowing interesting per-IO variant behaviour without having > to completely re-implemnt the IO path via custom ioctls every time > we want slightly different functionality... Al, Linus, what do you think? Is there a path forward for this series as is? I'd be happy to have this functionality merged in any form, but I do think that this approach with preadv2/pwritev2 using iov_len is decent relative to the alternatives.
