Re: [RFC PATCH] xfs: support for non-mmu architectures

From: Octavian Purdila
Date: Sun Nov 22 2015 - 20:41:56 EST

On Mon, Nov 23, 2015 at 12:44 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Sat, Nov 21, 2015 at 12:26:47AM +0200, Octavian Purdila wrote:
>> On Fri, Nov 20, 2015 at 11:08 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> > On Fri, Nov 20, 2015 at 03:43:20PM +0200, Octavian Purdila wrote:
>> >> On Fri, Nov 20, 2015 at 1:24 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> >> > On Wed, Nov 18, 2015 at 12:46:21AM +0200, Octavian Purdila wrote:
>> >> >> Naive implementation for non-mmu architectures: allocate physically
>> >> >> contiguous xfs buffers with alloc_pages. Terribly inefficient with
>> >> >> memory and fragmentation on high I/O loads but it may be good enough
>> >> >> for basic usage (which most non-mmu architectures will need).
>> >> >
>> >> > Can you please explain why you want to use XFS on low end, basic
>> >> > non-MMU devices? XFS is a high performance, enterprise/HPC level
>> >> > filesystem - it's not a filesystem designed for small IoT level
>> >> > devices - so I'm struggling to see why we'd want to expend any
>> >> > effort to make XFS work on such devices....
>> >> >
>> >>
>> >> Hi David,
>> >>
>> >> Yes XFS as the main fs on this type of devices does not make sense,
>> >> but does it hurt to be able to perform basic operation on XFS from
>> >> these devices? Perhaps accessing an external medium formatted with
>> >> XFS?
>> >>
>> >> Another example is accessing VM images that are formatted with XFS.
>> >> Currently we can do that with tools like libguestfs that use a VM in
>> >> the background. I am working on a lighter solution for that where we
>> >> compile the Linux kernel as a library [1]. This allows access to the
>> >> filesystem without the need to use a full VM.
>> >
>> > That's hardly a "lighter solution"
>> >
>> > I'm kinda tired of the ongoing "hack random shit" approach to
>> > container development.
>> Since apparently there is a container devs hunting party going on
>> right now, let me quickly confess that LKL has nothing to do with
>> (them be damned) containers :)
>> On a more serious note, LKL was not developed for containers or to try
>> to circumvent privileged mounts. It was developed to allow the Linux
>> kernel code to be reused in things like simple tools that allows one
>> to modify a filesystem image.
> Anything tool that modifies an XFS filesystem that is not directly
> maintained by the XFS developers voids any kind of support we can
> supply. Just like the fact we don't support tainted kernels because
> the 3rd party binary code is unknowable (and usually crap), having
> the kernel code linked with random 3rd party userspace application
> code is completely unsupportable by us.

Perhaps tainting the kernel is a solution when running unknown
applications linked with LKL.

I would argue that applications that are maintained together with LKL
(e.g. lklfuse in tools/lkl) should not taint the kernel because those
applications will be under the control of kernel developers. I would
also argue that mounting a filesystem read-only should not taint the
kernel either.

> Remember that with most kernel code a bug just results in a panic
> and reboot, and everything just continues on again after the system
> comes up again. In contrast, a bug in the storage code can cause
> *persistent damage* that can cause data loss or corruption that
> cannot be fixed without data loss of some kind.
> Ultimately, as the maintainer I'm responsible for XFS not eating our
> users' data, and part of that responsibility involves telling people
> who want to do daft things that "no, that's a *bad idea*".

I understand how critical filesystem issues are and I appreciate your
feedback. Sorry to drag you deeper into this but as you know, no good
deed goes unpunished :)

>> > If you need a XFS-FUSE module to allow safe
>> > userspace access to XFS fielsystems then maybe, just maybe, it makes
>> > sense to ask the XFS developers how to best go about providing a
>> > reliable, up-to-date, tested, maintained and supported XFS-FUSE
>> > module?
>> >
>> > IOWs, a "lighter solution" is to use the libxfs code base that we
>> > already maintain across kernel and userspace in the xfsprogs package
>> > and write a FUSE wrapper around that. That, immediately, will give
>> > you full read-only access to XFS filesystem images via FUSE. Then we
>> > (the XFS developers) can test the XFS-FUSE module under normal
>> > development conditions as we modify the xfsprogs code base (e.g. via
>> > xfstests) and ensure we always release a working, up-to-date FUSE
>> > wrapper with each xfsprogs release.
>> >
>> > And then once a proper read-only FUSE wrapper has been written, then
>> > we can discuss what is necessary to enable write access via porting
>> > the necessary parts of the kernel code across to the userspace
>> > libxfs codebase and hooking them up to the FUSE API...
>> >
>> > Hmmm?
>> >
>> What about ext4, vfat, btrfs and other filesystems?
> Ted has also raised exactly the same issues w.r.t. ext4.
>> Also why duplicate
>> the whole thing if you could reuse it?
> Do you use a hammer when you need to tighten a screw? Yes, you can
> "reuse a hammer" for this purpose, but there's going to be
> collateral damage because using the screw outside it's original
> design and architecture constraints presents a high risk of things
> going wrong.

First, lets try to discuss the potential collateral damage before
calling it a hammer. It may just be just an unfamiliar screw-driver :)

>> >> And a final example is linking the bootloader code with LKL to access
>> >> the filesystem. This has a hard requirement on non-mmu.
>> >
>> > No way. We *can't* support filesystems that have had bootloaders
>> > make arbitrary changes to the filesystem without the knowlege of the
>> > OS that *owns the filesystem*. Similarly, we cannot support random
>> > applications that internally mount and modify filesystem images in
>> > ways we can't see, control, test or simulate. Sure, they use the
>> > kernel code, but that doesn't stop them from doing stupid shit that
>> > could corrupt the filesystem image. So, no, we are not going to
>> > support giving random applications direct access to XFS filesystem
>> > images, even via LKL.
>> >
>> LKL only exports the Linux kernel system calls and nothing else to
>> applications. Because of that, there should not be any loss of control
>> or visibility to the XFS fs driver.
> It runs in the same address space as the user application, yes? And
> hence application bugs can cause the kernel code to malfunction,
> yes?

Most non-mmu architecture have the same issue and nevertheless non-mmu
is still supported in Linux (including most filesystems).

Also, filesystem code runs in the same address space with other kernel
code and drivers and a bug anywhere in the kernel can cause filesystem
code to malfunction. Applications maintained together with LKL and in
the kernel tree will be as safe as drivers and other kernel code with
regard to filesystem malfunctions. We can taint the kernel when LKL is
linked with unknown applications.

>> > I really don't see how using LKL to give userspace access to XFS
>> > filesystems is a better solution than actually writing a proper,
>> > supported XFS-FUSE module. LKL is so full of compromises that it's
>> > going to be unworkable and unsupportable in practice...
>> Could you elaborate on some of these issues?
> Start with "is a no-mmu architecture" and all the compromises that
> means the kernel code needs to make,

I don't see non-mmu as a compromise. It is supported by Linux and most
filesystems work fine on non-mmu architectures. LKL can be implemented
as a mmu architecture. Having it as a non-mmu architecture has the
advantages of allowing it to run in more constrained environments like

> add a topping of "runs in the
> same address space as the application",

I've addressed this concern above.

> add a new flavour of kernel
> binary taint

I am not sure I understand, are you saying that it is an issue to add
a new taint flavor?

> and finish it off with "LKL linked applications will
> never be tested by their developers over the full functionality the
> LKL provides them with".

You lost me here. Why does an application developer have to test the
full functionality of a library it is linked with?
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