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
