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Message-ID: <20250610190026.GA6134@frogsfrogsfrogs> Date: Tue, 10 Jun 2025 12:00:26 -0700 From: "Darrick J. Wong" <djwong@...nel.org> To: Amir Goldstein <amir73il@...il.com> Cc: linux-fsdevel <linux-fsdevel@...r.kernel.org>, John@...ves.net, bernd@...ernd.com, miklos@...redi.hu, joannelkoong@...il.com, Josef Bacik <josef@...icpanda.com>, linux-ext4 <linux-ext4@...r.kernel.org>, Theodore Ts'o <tytso@....edu> Subject: Re: [RFC[RAP]] fuse: use fs-iomap for better performance so we can containerize ext4 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@...nel.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@...nel.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@...nel.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? 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. > current fuse2fs code needs to go to an effort to convert from full path > to inode + name using ext2fs_namei(). > > With the low-level fuse op_lookup() might have used the native ext2_lookup() > which would have been much more natural. > > You can find the most featureful low-level fuse example at: > https://github.com/libfuse/libfuse/blob/master/example/passthrough_hp.cc > > Among other things, the server has an inode cache, where an inode > has in its state 'nopen' (was this inode opened for io) and 'backing_id' > (was this inode mapped for kernel passthrough). > > Currently this backing_id mapping is only made on first open of inode, > but the plan is to do that also at lookup time, for example, if the > iomap mode for the inode can be determined at lookup time. <nod> > > > > > > 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. > > > > Now that I have explained that fuse2fs should be low-level, it should be > trivial to claim that it should have no problem to declare via > FUSE_PASSTHROUGH_INO flag to the kernel that nodeid == st_ino, > because I see no reason to implement fuse2fs with non one-to-one > mapping of ino <==> nodeid. Agreed! Thanks for the nudge! Let's see what Ted thinks when he returns from vacation. :) --D
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