Re: [PATCH] udmabuf: revert 'Add support for mapping hugepages (v4)'

[David Hildenbrand <david@redhat.com>]

>> Skimming over at shmem_read_mapping_page() users, I assume most of
>> them
>> use a VM_PFNMAP mapping (or don't mmap them at all), where we won't be
>> messing with the struct page at all.
>>
>> (That might even allow you to mmap hugetlb sub-pages, because the struct
>> page -- and mapcount -- will be ignored completely and not touched.)
> Oh, are you suggesting that if we do vma->vm_flags |= VM_PFNMAP
> in the mmap handler (mmap_udmabuf) and also do
> vmf_insert_pfn(vma, vmf->address, page_to_pfn(page))
> instead of
> vmf->page = ubuf->pages[pgoff];
> get_page(vmf->page);
> 
> in the vma fault handler (udmabuf_vm_fault), we can avoid most of the
> pitfalls you have identified -- including with the usage of hugetlb subpages?

Yes, that's my thinking, but I have to do my homework first to see if 
that would really work for hugetlb.

The thing is, I kind-of consider what udmabuf does a layer violation: we 
have a filesystem (shmem/hugetlb) that should handle mappings to user 
space. Yet, a driver decides to bypass that and simply map the pages 
ordinarily to user space. (revealed by the fact that hugetlb does never 
map sub-pages but udmabuf decides to do so)

In an ideal world everybody would simply mmap() the original memfd, but 
thinking about offset+size configuration within the memfd that might not 
always be desirable. As a workaround, we could mmap() only the PFNs, 
leaving the struct page unaffected.

I'll have to look closer into that.

-- 
Cheers,

David / dhildenb