On Tue, Aug 27, 2024 at 3:48 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Tue, Aug 27, 2024 at 11:42 AM Han-Wen Nienhuys <hanwenn@xxxxxxxxx> wrote: > > > > Hi folks, > > Hi Han-Wen, > > For future reference, please CC FUSE and libfuse maintainers > and FUSE passthrough developer (me) if you want to make sure > that you got our attention. Sure. Who is the libfuse maintainer these days? > > I tried simply closing the backing FD right after obtaining the > > backing ID, and it seems to work. Is this permitted? > > Yes. awesome! > BTW, since you are one of the first (publicly announced) users of > FUSE passthrough, it would be helpful to get feedback about API, > which could change down the road and about your wish list. For full transparency, I just work on the go-fuse library for fun, I don't have direct applications for passthrough at the moment. That said, I have been trying to make it go faster, and obviously bypassing user-space for reads/writes helps in a big way. > Specifically, I have WIP patches for > - readdir() passthrough > - stat()/getxattr()/listxattr() passthrough > > and users feedback could help me decide how to prioritize between > those (and other) FUSE passthrough efforts. It's been useful in the past to defer all file operations to an underlying file, and only change where in the tree a file is surfaced. In that sense, it would be nice to just say "pass all operations on this (open) file to this other file". Go-FUSE has a LoopbackFile and LoopbackNode that lets you do that very easily, but it would be extra attractive if that would also avoid kernel roundtrips. For flexibility, it might be useful to pass a bitmask or a list of FUSE opcodes back to the kernel, so you can select which operations should be passed through. The most annoying part of the current functionality is the CAP_SYS_ADMIN restriction; I am not sure everyone is prepared to run their file systems as root. Could the ioctl check that the file was opened as O_RDWR, and stop checking for root? If you are proposing to do xattr functions, that means that you also will do passthrough for files that are not opened? Right now, the backing ID is in the OpenOut structure; how would you pass back the backing ID so it could be used for stat() ? IMO the most natural solution would be to extend the lookup response so you can directly insert a backing ID there. I made a mistake in how I treat opendir/readdir in the high-level go-fuse API. Until that is fixed, I cannot use passthrough for readdir. That said, part of making FUSE filesystems go fast is to amortize the cost of lookup using readdirplus. That wouldn't work with readdir passthrough, so it may not be that useful. So in summary, my wishlist for passthrough (decreasing importance): 1. get rid of cap_sys_admin requirement 2. allow passthrough for all file operations (fallocate, fsync, flush, setattr, etc.) cheers, -- Han-Wen Nienhuys - hanwenn@xxxxxxxxx - http://www.xs4all.nl/~hanwen
