Re: [RFC[RAP]] fuse: use fs-iomap for better performance so we can containerize ext4

Amir Goldstein
2025-06-10T19:52:10Z

On Tue, Jun 10, 2025 at 9:00 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> On Tue, Jun 10, 2025 at 12:59:36PM +0200, Amir Goldstein wrote:
> > On Tue, Jun 10, 2025 at 12:32 AM Darrick J. Wong <djwong@kernel.org> wrote:
> > >
> > > On Thu, May 29, 2025 at 09:41:23PM +0200, Amir Goldstein wrote:
> > > > or
> > > >
> > > > On Thu, May 29, 2025 at 6:45 PM Darrick J. Wong <djwong@kernel.org> wrote:
> > > > >
> > > > > On Thu, May 22, 2025 at 06:24:50PM +0200, Amir Goldstein wrote:
> > > > > > On Thu, May 22, 2025 at 1:58 AM Darrick J. Wong <djwong@kernel.org> wrote:
> > > > > > >
> > > > > > > Hi everyone,
> > > > > > >
> > > > > > > DO NOT MERGE THIS.
> > > > > > >
> > > > > > > This is the very first request for comments of a prototype to connect
> > > > > > > the Linux fuse driver to fs-iomap for regular file IO operations to and
> > > > > > > from files whose contents persist to locally attached storage devices.
> > > > > > >
> > > > > > > Why would you want to do that? Most filesystem drivers are seriously
> > > > > > > vulnerable to metadata parsing attacks, as syzbot has shown repeatedly
> > > > > > > over almost a decade of its existence. Faulty code can lead to total
> > > > > > > kernel compromise, and I think there's a very strong incentive to move
> > > > > > > all that parsing out to userspace where we can containerize the fuse
> > > > > > > server process.
> > > > > > >
> > > > > > > willy's folios conversion project (and to a certain degree RH's new
> > > > > > > mount API) have also demonstrated that treewide changes to the core
> > > > > > > mm/pagecache/fs code are very very difficult to pull off and take years
> > > > > > > because you have to understand every filesystem's bespoke use of that
> > > > > > > core code. Eeeugh.
> > > > > > >
> > > > > > > The fuse command plumbing is very simple -- the ->iomap_begin,
> > > > > > > ->iomap_end, and iomap ioend calls within iomap are turned into upcalls
> > > > > > > to the fuse server via a trio of new fuse commands. This is suitable
> > > > > > > for very simple filesystems that don't do tricky things with mappings
> > > > > > > (e.g. FAT/HFS) during writeback. This isn't quite adequate for ext4,
> > > > > > > but solving that is for the next sprint.
> > > > > > >
> > > > > > > With this overly simplistic RFC, I am to show that it's possible to
> > > > > > > build a fuse server for a real filesystem (ext4) that runs entirely in
> > > > > > > userspace yet maintains most of its performance. At this early stage I
> > > > > > > get about 95% of the kernel ext4 driver's streaming directio performance
> > > > > > > on streaming IO, and 110% of its streaming buffered IO performance.
> > > > > > > Random buffered IO suffers a 90% hit on writes due to unwritten extent
> > > > > > > conversions. Random direct IO is about 60% as fast as the kernel; see
> > > > > > > the cover letter for the fuse2fs iomap changes for more details.
> > > > > > >
> > > > > >
> > > > > > Very cool!
> > > > > >
> > > > > > > There are some major warts remaining:
> > > > > > >
> > > > > > > 1. The iomap cookie validation is not present, which can lead to subtle
> > > > > > > races between pagecache zeroing and writeback on filesystems that
> > > > > > > support unwritten and delalloc mappings.
> > > > > > >
> > > > > > > 2. Mappings ought to be cached in the kernel for more speed.
> > > > > > >
> > > > > > > 3. iomap doesn't support things like fscrypt or fsverity, and I haven't
> > > > > > > yet figured out how inline data is supposed to work.
> > > > > > >
> > > > > > > 4. I would like to be able to turn on fuse+iomap on a per-inode basis,
> > > > > > > which currently isn't possible because the kernel fuse driver will iget
> > > > > > > inodes prior to calling FUSE_GETATTR to discover the properties of the
> > > > > > > inode it just read.
> > > > > >
> > > > > > Can you make the decision about enabling iomap on lookup?
> > > > > > The plan for passthrough for inode operations was to allow
> > > > > > setting up passthough config of inode on lookup.
> > > > >
> > > > > The main requirement (especially for buffered IO) is that we've set the
> > > > > address space operations structure either to the regular fuse one or to
> > > > > the fuse+iomap ops before clearing INEW because the iomap/buffered-io.c
> > > > > code assumes that cannot change on a live inode.
> > > > >
> > > > > So I /think/ we could ask the fuse server at inode instantiation time
> > > > > (which, if I'm reading the code correctly, is when iget5_locked gives
> > > > > fuse an INEW inode and calls fuse_init_inode) provided it's ok to upcall
> > > > > to userspace at that time. Alternately I guess we could extend struct
> > > > > fuse_attr with another FUSE_ATTR_ flag, I think?
> > > > >
> > > >
> > > > The latter. Either extend fuse_attr or struct fuse_entry_out,
> > > > which is in the responses of FUSE_LOOKUP,
> > > > FUSE_READDIRPLUS, FUSE_CREATE, FUSE_TMPFILE.
> > > > which instantiate fuse inodes.
> > > >
> > > > There is a very hand wavy discussion about this at:
> > > > https://lore.kernel.org/linux-fsdevel/CAOQ4uxi2w+S4yy3yiBvGpJYSqC6GOTAZQzzjygaH3TjH7Uc4+Q@mail.gmail.com/
> > > >
> > > > In a nutshell, we discussed adding a new FUSE_LOOKUP_HANDLE
> > > > command that uses the variable length file handle instead of nodeid
> > > > as a key for the inode.
> > > >
> > > > So we will have to extend fuse_entry_out anyway, but TBH I never got to
> > > > look at the gritty details of how best to extend all the relevant commands,
> > > > so I hope I am not sending you down the wrong path.
> > >
> > > I found another twist to this story: the upper level libfuse3 library
> > > assigns distinct nodeids for each directory entry. These nodeids are
> > > passed into the kernel and appear to the basis for an iget5_locked call.
> > > IOWs, each nodeid causes a struct fuse_inode to be created in the
> > > kernel.
> > >
> > > For a single-linked file this is no big deal, but for a hardlink this
> > > makes iomap a mess because this means that in fuse2fs, an ext2 inode can
> > > map to multiple kernel fuse_inode objects. This /really/ breaks the
> > > locking model of iomap, which assumes that there's one in-kernel inode
> > > and that it can use i_rwsem to synchronize updates.
> > >
> > > So I'm going to have to find a way to deal with this. I tried trivially
> > > messing with libfuse nodeid assigment but that blew some assertion.
> > > Maybe your LOOKUP_HANDLE thing would work.
> > >
> >
> > Pull the emergency break!
> >
> > In an amature move, I did not look at fuse2fs.c before commenting on your
> > work.
> >
> > High level fuse interface is not the right tool for the job.
> > It's not even the easiest way to have written fuse2fs in the first place.
>
> At the time I thought it would minimize friction across multiple
> operating systems' fuse implementations.
>
> > High-level fuse API addresses file system objects with full paths.
> > This is good for writing simple virtual filesystems, but it is not the
> > correct nor is the easiest choice to write a userspace driver for ext4.
>
> Agreed, it's a *terrible* way to implement ext4.
>
> I think, however, that Ted would like to maintain compatibility with
> macfuse and freebsd(?) so he's been resistant to rewriting the entire
> program to work with the lowlevel library.
>
> That said, I decided just now to do some spelunking into those two fuse
> ports and have discovered that freebsd[1] packages the same upstream
> libfuse as linux, and macfuse[2] seems to vendor both libfuse 2 and 3.
>
> [1] https://wiki.freebsd.org/FUSEFS
> [2] https://github.com/macfuse/macfuse
>
> Seeing as Debian 13 has killed off libfuse2 entirely, maybe I should
> think about rewriting all of fuse2fs against the lowlevel library? It's
> really annoying to deal with all the problems of the current codebase.
> I think I'll try to stabilize the current fuse+iomap code and then look
> into a fuse2fs port. What would we call it, fuse4fs? :D
>
> > Low-level fuse interface addresses filesystem objects by nodeid
> > and requires the server to implement lookup(parent_nodeid, name)
> > where the server gets to choose the nodeid (not libfuse).
>
> Does the nodeid for the root directory have to be FUSE_ROOT_ID?

Yeh, I think that's the case, otherwise FUSE_INIT would need to
tell the kernel the root nodeid, because there is no lookup to
return the root nodeid.

> I guess
> for ext4 that's not a big deal since ext2 inode #1 is the badblocks file
> which cannot be accessed from userspace anyway.
>

As long as inode #1 is reserved it should be fine.
just need to refine the rules of the one-to-one mapping with
this exception.

Thanks,
Amir.