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 15:31, Pavel Begunkov wrote:
> 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.

FWIW, the easiest solution is to internally reuse regions for
allocations and mmap()'ing and wrap it into a registered buffer.
It just need to make vmap'ing optional as it won't be needed.

-- 
Pavel Begunkov