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

["Darrick J. Wong" <djwong@kernel.org>]

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.

> > > > 5. ext4 doesn't support out of place writes so I don't know if that
> > > > actually works correctly.
> > > >
> > > > 6. iomap is an inode-based service, not a file-based service.  This
> > > > means that we /must/ push ext2's inode numbers into the kernel via
> > > > FUSE_GETATTR so that it can report those same numbers back out through
> > > > the FUSE_IOMAP_* calls.  However, the fuse kernel uses a separate nodeid
> > > > to index its incore inode, so we have to pass those too so that
> > > > notifications work properly.
> > > >
> > >
> > > Again, I might be missing something, but as long as the fuse filesystem
> > > is exposing a single backing filesystem, it should be possible to make
> > > sure (via opt-in) that fuse nodeid's are equivalent to the backing fs
> > > inode number.
> > > See sketch in this WIP branch:
> > > https://github.com/amir73il/linux/commit/210f7a29a51b085ead9f555978c85c9a4a503575
> >
> > I think this would work in many places, except for filesystems with
> > 64-bit inumbers on 32-bit machines.  That might be a good argument for
> > continuing to pass along the nodeid and fuse_inode::orig_ino like it
> > does now.  Plus there are some filesystems that synthesize inode numbers
> > so tying the two together might not be feasible/desirable anyway.
> >
> > Though one nice feature of letting fuse have its own nodeids might be
> > that if the in-memory index switches to a tree structure, then it could
> > be more compact if the filesystem's inumbers are fairly sparse like xfs.
> > OTOH the current inode hashtable has been around for a very long time so
> > that might not be a big concern.  For fuse2fs it doesn't matter since
> > ext4 inumbers are u32.
> >
> 
> I wanted to see if declaring one-to-one 64bit ino can simplify things
> for the first version of inode ops passthrough.
> If this is not the case, or if this is too much of a limitation for
> your use case
> then nevermind.
> But if it is a good enough shortcut for the demo and can be extended later,
> then why not.

It's very tempting, because it's very confusing to have nodeids and
stat st_ino not be the same thing.

--D

> Thanks,
> Amir.
>