Re: [PATCH] RDMA/uverbs: Don't return the shared zero page from rdma_umap_fault()

[Zi Yan]

On 28 Oct 2026, at 4:43, David Hildenbrand (Arm) wrote:

>>>> +	/*
>>>> +	 * VMAs that cannot become writable map the page as a raw pfn, like
>>>> +	 * the device memory it replaces. The page lives as long as the file.
>>>> +	 */
>>>> +	if (!(vmf->vma->vm_flags & (VM_WRITE | VM_MAYWRITE))) {
>>>> +		ret = vmf_insert_pfn(vmf->vma, vmf->address, page_to_pfn(page));
>>>> +		goto out;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * This VMA is forced to always be shared so this doesn't have
>>>> +	 * to worry about COW.
>>>> +	 */
>>>> +	vmf->page = page;
>>>> +	get_page(page);
>>>
>>>
>>> This split handling is pretty horrible TBH.
>>>
>>> In once case we take a reference in the other not?
>>>
>>> Why not perform a PFN mapping in any case?
>>
>> For writable path, a new vmf_insert_pfn_mkwrite() is needed, so it was
>> not fixed. I can fix that part as well.
>
> We should do that; returning kernel allocations from this function in ->fault is
> bad.
>
> How urgent is this fix here? (can it actually be triggered?Specific hardware?)
>

Not urgent. Like Jason said in another email, RDMA only runs on arch
with special PTE, so refcount, mapcount, rss are balanced. With DEBUG_VM,
zero_page checks in rmap will fire on the returned zero_page.

>>
>> The read-only zero page part might need more attention, since
>> we just prevented vmf_insert_pfn_prot() from using zero_page. Do we want
>> to add the support for rdma_umap_fault()?
>
> We just have to make sure that we cannot install it writable etc. But that's not
> a concern.
>
> Just have to be careful :)

There is one wrinkle, it does not work on
s390 + mm_forbids_zeropage() == true, where zero page is not present,
and a private zero page is needed.

Jason, is RDMA supported on s390? If yes, it is better to
let the rdma driver to allocate a driver visible zero page to share
across ufiles. So we do not need separate handlings for s390 and
others. Let me know your thoughts on this.

>
>>
>>>
>>> (Does GUP have to work on these things?)
>>
>> These are VM_PFNMAP mappings, I guess not. But I am not a GUP expert.
>
> So we installed folios with ->fault in VM_PFNMAP!?!?!?!
>
> That doesn't sound right :)



Best Regards,
Yan, Zi