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

[Jens Axboe <axboe@kernel.dk>]

On 4/22/26 7:03 AM, Greg Kroah-Hartman wrote:
> On Wed, Apr 22, 2026 at 06:40:37AM -0600, Jens Axboe wrote:
>> 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.
> 
> No objection at all, looks fine, thanks.
> 
> And sorry for this, no-mmu is odd, hopefully the 3 users of it and
> io_uring are thankful for this work :)

Basically everything we do in the kernel is a thankless task, so I'm not
expecting someone to send me a box of chocolates. But at least it should
be sorted now!

-- 
Jens Axboe