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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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 :)

greg k-h