Re: [PATCH] io_uring: take page references for NOMMU pbuf_ring mmaps

[Jens Axboe <axboe@kernel.dk>]

On 4/22/26 2:11 AM, Greg Kroah-Hartman wrote:
> On Tue, Apr 21, 2026 at 08:26:08PM -0600, Jens Axboe wrote:
>> On 4/21/26 7:56 PM, Jens Axboe wrote:
>>> On 4/21/26 7:17 PM, Jens Axboe wrote:
>>>> On 4/21/26 11:39 AM, Jens Axboe wrote:
>>>>>
>>>>> On Tue, 21 Apr 2026 15:46:16 +0200, Greg Kroah-Hartman wrote:
>>>>>> Under !CONFIG_MMU, io_uring_get_unmapped_area() returns the kernel
>>>>>> virtual address of the io_mapped_region's backing pages directly;
>>>>>> the user's VMA aliases the kernel allocation. io_uring_mmap() then
>>>>>> just returns 0 -- it takes no page references.
>>>>>>
>>>>>> The CONFIG_MMU path uses vm_insert_pages(), which takes a reference on
>>>>>> each inserted page.  Those references are released when the VMA is torn
>>>>>> down (zap_pte_range -> put_page). io_free_region() -> release_pages()
>>>>>> drops the io_uring-side references, but the pages survive until munmap
>>>>>> drops the VMA-side references.
>>>>>>
>>>>>> [...]
>>>>>
>>>>> Applied, thanks!
>>>>>
>>>>> [1/1] io_uring: take page references for NOMMU pbuf_ring mmaps
>>>>>       commit: d9b7b3d9c5286a786c7fe8220c55a6e012088c2e
>>>>
>>>> Actually, I take that back - what prevents the io_mmap_get_region()
>>>> in the newly added io_uring_nommu_vm_close() from getting the same
>>>> region that we initially referenced the pages from in the nommu
>>>> variant of io_uring_mmap()?
>>>
>>> I think we can get rid of that and simplify the code at the same
>>> time. Rather than need to re-lookup the buffer list, we can just iterate
>>> the pages mapped in the vma. Since this is a file backed mapping and
>>> io_uring doesn't allow remaps, that should always be the same.
>>>
>>> Greg, can you test this? I will fold this in.
>>
>> Here's the full patch - the incremental was missing a ')'. And
>> for good measure, ensure that the vma size matches the pages in
>> the region.
> 
> Yes, this works, thanks!
> 
> Tested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

I kept your attribution with the changes folded in, it's here
since last night:

https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git/commit/?h=io_uring-7.1&id=d0be8884f56b0b800cd8966e37ce23417cd5044e

Let me know if that's still fine with you.

-- 
Jens Axboe