Re: [RFC PATCH 03/24] erofs: add Errno in Rust
From: Gao Xiang
Date: Thu Sep 19 2024 - 20:49:53 EST
On 2024/9/20 03:36, Benno Lossin wrote:
On 19.09.24 17:13, Gao Xiang wrote:
Hi Benno,
On 2024/9/19 21:45, Benno Lossin wrote:
Hi,
Thanks for the patch series. I think it's great that you want to use
Rust for this filesystem.
On 17.09.24 01:58, Gao Xiang wrote:
On 2024/9/17 04:01, Gary Guo wrote:
Also, it seems that you're building abstractions into EROFS directly
without building a generic abstraction. We have been avoiding that. If
there's an abstraction that you need and missing, please add that
abstraction. In fact, there're a bunch of people trying to add FS
No, I'd like to try to replace some EROFS C logic first to Rust (by
using EROFS C API interfaces) and try if Rust is really useful for
a real in-tree filesystem. If Rust can improve EROFS security or
performance (although I'm sceptical on performance), As an EROFS
maintainer, I'm totally fine to accept EROFS Rust logic landed to
help the whole filesystem better.
As Gary already said, we have been using a different approach and it has
served us well. Your approach of calling directly into C from the driver
can be used to create a proof of concept, but in our opinion it is not
something that should be put into mainline. That is because calling C
from Rust is rather complicated due to the many nuanced features that
Rust provides (for example the safety requirements of references).
Therefore moving the dangerous parts into a central location is crucial
for making use of all of Rust's advantages inside of your code.
I'm not quite sure about your point honestly. In my opinion, there
is nothing different to use Rust _within a filesystem_ or _within a
driver_ or _within a Linux subsystem_ as long as all negotiated APIs
are audited.
To us there is a big difference: If a lot of functions in an API are
`unsafe` without being inherent from the problem that it solves, then
it's a bad API.
Which one? If you point it out, we will update the EROFS kernel
APIs then.
Otherwise, it means Rust will never be used to write Linux core parts
such as MM, VFS or block layer. Does this point make sense? At least,
Rust needs to get along with the existing C code (in an audited way)
rather than refuse C code.
I am neither requiring you to write solely safe code, nor am I banning
interacting with the C side. What we mean when we talk about
abstractions is that we want to minimize the Rust code that directly
interfaces with C. Rust-to-Rust interfaces can be a lot safer and are
We will definitly minimize the API interface between Rust and C in
EROFS.
And it can be done incrementally, why not? I assume your world is
not pure C and pure Rust as for the Rust for Linux project, no?
easier to implement correctly.
My personal idea about Rust: I think Rust is just another _language
tool_ for the Linux kernel which could save us time and make the
kernel development better.
Yes, but we do have conventions, rules and guidelines for writing such
code. C code also has them. If you want/need to break them, there should
be a good reason to do so. I don't see one in this instance.
>> Or I wonder why not writing a complete new Rust stuff instead rather
than living in the C world?
There are projects that do that yes. But Rust-for-Linux is about
bringing Rust to the kernel and part of that is coming up with good
conventions and rules.
Which rule is broken? Was they discussed widely around the
Linux world?
For Rust VFS abstraction, that is a different and indepenent story,
Yiyang don't have any bandwidth on this due to his limited time.
This seems a bit weird, you have the bandwidth to write your own
abstractions, but not use the stuff that has already been developed?
It's not written by me, Yiyang is still an undergraduate tudent.
It's his research project and I don't think it's his responsibility
to make an upstreamable VFS abstraction.
That is fair, but he wouldn't have to start from scratch, Wedsons
abstractions were good enough for him to write a Rust version of ext2.
The Wedson one is just broken, I assume that you've read
https://lwn.net/Articles/978738/ ?
The initial Linux VFS C version is already for generic
read-write use.
In addition, tarfs and puzzlefs also use those bindings.
These are both toy fses, I don't know who will use these two
fses for their customers.
To me it sounds as if you have not taken the time to try to make it work
with the existing abstractions. Have you tried reaching out to Ariel? He
is working on puzzlefs and might have some insight to give you. Sadly
IMHO, puzzlefs is another Rust incomplete clone of EROFS, I
could tell him what EROFS currently do.
I'm very happy to collaborate with him to work on his use
cases (and tell him why EROFS can already be used for his
use cases), just
like the previous ComposeFS discussion.
There are enough FS projects which reinvents in-tree fses without
enough good reasons (for example, performance or design): ZUFS,
FamFS, ComposeFS.
Tarfs (here tar is not the real tar format), and Puzzlefs are
two special one just because they are written in Rust. But
other than that they are just incomplete approach to EROFS.
I do hope Ariel could attend LSF/MM/BPF to discuss his use
cases with filesystem developpers. And I'm very happy to
collaborate with him .
Wedson has left the project, so someone will have to pick up his work.
It's not necessary to be Yiyang, since he's interested in
EROFS only.
I hope that you understand that we can't have two abstractions for the
same C API. It confuses people which to use, some features might only be
available in one version and others only in the other. It would be a
total mess. It's just like the rule for no duplicated drivers that you
have on the C side.
People (mostly Wedson) also have put in a lot of work into making the
VFS abstractions good. Why ignore all of that?
How good? TBH, I think there could be something left eventually,
but the current prososed Rust VFS abstraction is just broken.
I don't think the current abstraction is of any use to be
upstreamed, at least, it should be driven with a generic
read-write filesystem, and resolve lifetime issues during
development (for example, just like Al mentioned d_name and
d_parent, etc.)
Because the initial Linux VFS C version is completely out of
minix fs, rather than an incomplete broken one just for some
toy (I don't know how to express more accurately, since each
upstream filesystem should have strong use cases and users,
but tarfs and puzzlefs are both not.)
Otherwise, all the broken Rust VFS users will be painful for
bugs and endless API refactering.
I have quickly glanced over the patchset and the abstractions seem
rather immature, not general enough for other filesystems to also take
I don't have enough time to take a full look of this patchset too
due to other ongoing work for now (Rust EROFS is not quite a high
priority stuff for me).
And that's why it's called "RFC PATCH".
Yeah I saw the RFC title. I just wanted to communicate early that I
would not review it if it were a normal patch. In fact, I would advise
against taking the patch, due to the reasons I outlined.
You reason currently is still not valid. IMO, again, Rust
is just a tool, you cannot forbid a real Linux subsystem to
use Rust as an experiment.
Or what's your real point? An Rust gatekeeper?
advantage of them. They also miss safety documentation and are in
I don't think it needs to be general enough, since we'd like to use
the new Rust language tool within a subsystem.
So why it needs to take care of other filesystems? Again, I'm not
working on a full VFS abstriction.
And that's OK, feel free to just pick the parts of the existing VFS that
you need and extend as you (or your student) see fit. What you said
yourself is that we need a global vision for VFS abstractions. If you
only use a subset of them, then you only care about that subset, other
people can extend it if they need. If everyone would roll their own
abstractions without communicating, then how would we create a global
vision?
No. We don't roll our own abstraction, instead we define a
clear C <-> Rust boundary of EROFS APIs, just like
"fs/xfs/libxfs" if you could take a look.
Yes, this patchset is not perfect. But I've asked Yiyang to isolate
all VFS structures as much as possible, but it seems that it still
touches something.
It would already be a big improvement to put the VFS structures into the
kernel crate. Because then everyone can benefit from your work.
Again, that is not Yiyang's interest. Which is just like to sell
something you don't want, I don't think it's reasonable.
general poorly documented.
Okay, I think it can be improved then if you give more detailed hints.
Additionally, all of the code that I saw is put into the `fs/erofs` and
`rust/erofs_sys` directories. That way people can't directly benefit
from your code, put your general abstractions into the kernel crate.
Soon we will be split the kernel crate, I could imagine that we end up
with an `fs` crate, when that happens, we would put those abstractions
there.
As I don't have the bandwidth to review two different sets of filesystem
abstractions, I can only provide you with feedback if you use the
existing abstractions.
I think Rust is just a tool, if you could have extra time to review
our work, that would be wonderful! Many thanks then.
However, if you don't have time to review, IMHO, Rust is just a tool,
I think each subsystem can choose to use Rust in their codebase, or
I'm not sure what's your real point is?
I don't want to prevent or discourage you from using Rust in the kernel.
In fact, I can't prevent you from putting this in, since after all you
are the maintainer.
I do think you're discouraging anyone to use Rust in their codebase,
because I've said we _will_ form a good abstraction in our codebase.
But you're just selling another stuff forcely.
What I can do, is advise against not using abstractions. That has been
our philosophy since very early on. They are the reason that you can
write PHY drivers without any `unsafe` code whatsoever *today*. I think
I don't think filesystems are comparable to some PHY drivers. If you
take ext4, it's more than 65000 line, and XFS, that is almost 78000.
I think filesystems can have a way to be reimplmented in Rust
incrementally, rather than purely black and write world.
that is an impressive feat and our recipe for success.
We even have this in our documentation:
https://docs.kernel.org/rust/general-information.html#abstractions-vs-bindings
My real point is that I want Rust to succeed in the kernel. I strongly
believe that good abstractions (in the sense that you can do as much as
possible using only safe Rust) are a crucial factor.
On my side, you are just isolating any useful subsystem to try
to use Rust and
“Leaf” modules (e.g. drivers) should not use the C bindings directly
is unreasonable for filesystems because it cannot be done in one
shot.
In addition, there are a lot ongoing features on both C and Rust
side, you need at least a fallback to make end users happy with a
unique feature view rather than just return "it's broken".
Users don't care C or Rust (they only care full functionality)
but only developers care. And mixing up two different things
(VFS abstraction and use Rust in the codebase) is not good to
RFL success.
IMHO, this is "Rust for Linux" not "Linux for Rust".
I and others from the RfL team can help you if you (or your student)
have any Rust related questions for the abstractions. Feel free to reach
out.
Maybe Miguel can say more on this matter, since he was at the
maintainers summit, but our takeaways essentially are that we want
maintainers to experiment with Rust. And if you don't have any real
users, then breaking the Rust code is fine.
Though I think that with breaking we mean that changes to the C side
prevent the Rust side from working, not shipping Rust code without
abstractions.
I think you're still mixing them up.
We might be able to make an exception to the "your driver can only use
abstractions" rule, but only with the promise that the subsystem is
working towards creating suitable abstractions and replacing the direct
C accesses with that.
I don't think those rules are reasonable for RFL success, honestly.
You are artificially isolating the Linux C and Rust world, not from
Linux users or Linux ecosystem perspective, but only from some
developer perspersive.
Good luck, anyway.
I personally think that we should not make that the norm, instead try to
create the minimal abstraction and minimal driver (without directly
calling C) that you need to start. Of course this might not work, the
"minimal driver" might need to be rather complex for you to start, but I
don't know your subsystem to make that judgement.
...
Without a full proper VFS abstraction, it's just broken and
needs to be refactored. And that will be painful to all
users then.
I also don't understand your point here. What is broken, this EROFS
implementation? Why will it be painful to refactor?
I've said earlier.
=======
In the end,
Other thoughts, comments are helpful here since I wonder how "Rust
-for-Linux" works in the long term, and decide whether I will work
on Kernel Rust or not at least in the short term.
The longterm goal is to make everything that is possible in C, possible
in Rust. For more info, please take a look at the kernel summit talk by
But you're disallowing Rust in the codebase.
Miguel Ojeda.
However, we can only reach that longterm goal if maintainers are willing
and ready to put Rust into their subsystems (either by knowing/learning
Rust themselves or by having a co-maintainer that does just the Rust
part). So you wanting to experiment is great. I appreciate that you also
have a student working on this. Still, I think we should follow our
guidelines and create abstractions in order to require as little
`unsafe` code as possible.
I've expressed my point. I don't think some `guideline`
could bring success to RFL. Since many subsystems needs
an incremental way, not just a black-or-white thing.
Thanks,
Gao Xiang
---
Cheers,
Benno