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

[Octavian Purdila]

On Fri, Nov 20, 2015 at 5:24 PM, Brian Foster <bfoster@xxxxxxxxxx> wrote:
> On Fri, Nov 20, 2015 at 04:26:28PM +0200, Octavian Purdila wrote:
>> On Fri, Nov 20, 2015 at 2:58 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> > On Fri, Nov 20, 2015 at 12:54:02AM +0100, Richard Weinberger wrote:
>> >> On Fri, Nov 20, 2015 at 12:24 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....
>> >>
>> >> The use case is the Linux Kernel Library:
>> >> https://lkml.org/lkml/2015/11/3/706
>> >>
>> >> Using LKL and fuse you can mount any kernel filesystem using fuse
>> >> as non-root.
>> >
>> > IOWs, because we said no to unprivileged mounts, instead the
>> > proposal is to linking all the kernel code into userspace so you can
>> > do unprivielged mounts that way?
>> >
>>
>> LKL's goal is to make it easy for various applications to reuse Linux
>> kernel code instead of re-implementing it. Mounting filesystem images
>> is just one of the applications.
>>
>> > IOWs, you get to say "it secure because it's in userspace" and leave
>> > us filesystem people with all the shit that comes with allowing
>> > users to mount random untrusted filesystem images using code that
>> > was never designed to allow that to happen?
>> >
>>
>> It is already possible to mount arbitrary filesystem images in
>> userspace using VMs . LKL doesn't change that, it just reduces the
>> amount of dependencies you need to do so.
>>
>
> Perhaps a dumb question, but I'm not quite putting 2+2 together here.
> When I see nommu, I'm generally thinking hardware characteristics, but
> we're talking about a userspace kernel library here. So can you
> elaborate on how this relates to nommu? Does this library emulate kernel
> mechanisms in userspace via nommu mode or something of that nature?
>

LKL is currently implemented as a virtual non-mmu architecture. That
makes it simpler and it will also allow us to support environments
where it is not possible to emulate paging (e.g.  bootloaders).
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