Re: [PATCH v1 03/11] io_uring/kbuf: add support for kernel-managed buffer rings

[Pavel Begunkov <asml.silence@gmail.com>]

On 2/13/26 07:27, Christoph Hellwig wrote:
> On Thu, Feb 12, 2026 at 09:29:31AM -0800, Joanne Koong wrote:
>>>> I'm arguing exactly against this.  For my use case I need a setup
>>>> where the kernel controls the allocation fully and guarantees user
>>>> processes can only read the memory but never write to it.  I'd love
>>
>> By "control the allocation fully" do you mean for your use case, the
>> allocation/setup isn't triggered by userspace but is initiated by the
>> kernel (eg user never explicitly registers any kbuf ring, the kernel
>> just uses the kbuf ring data structure internally and users can read
>> the buffer contents)? If userspace initiates the setup of the kbuf
>> ring, going through IORING_REGISTER_MEM_REGION would be semantically
>> the same, except the buffer allocation by the kernel now happens
>> before the ring is created and then later populated into the ring.
>> userspace would still need to make an mmap call to the region and the
>> kernel could enforce that as read-only. But if userspace doesn't
>> initiate the setup, then going through IORING_REGISTER_MEM_REGION gets
>> uglier.
> 
> The idea is that the application tells the kernel that it wants to use
> a fixed buffer pool for reads.  Right now the application does this
> using io_uring_register_buffers().  The problem with that is that
> io_uring_register_buffers ends up just doing a pin of the memory,
> but the application or, in case of shared memory, someone else could
> still modify the memory.  If the underlying file system or storage
> device needs verify checksums, or worse rebuild data from parity
> (or uncompress), it needs to ensure that the memory it is operating
> on can't be modified by someone else.
> 
> So I've been thinking of a version of io_uring_register_buffers where
> the buffers are not provided by the application, but instead by the
> kernel and mapped into the application address space read-only for
> a while, and I thought I could implement this on top of your series,
> but I have to admit I haven't really looked into the details all
> that much.

There is nothing about registered buffers in this series. And even
if you try to reuse buffer allocation out of it, it'll come with
a circular buffer you'll have no need for. And I'm pretty much
arguing about separating those for io_uring.

-- 
Pavel Begunkov