On Fri, Nov 20, 2020 at 04:01:33PM -0400, Jason Gunthorpe wrote:
On Wed, Nov 11, 2020 at 03:38:42PM -0800, Ralph Campbell wrote:
MEMORY_DEVICE_GENERIC: Struct pages are created in dev_dax_probe() and represent non-volatile memory. The device can be mmap()'ed which calls dax_mmap() which sets vma->vm_flags | VM_HUGEPAGE. A CPU page fault will result in a PTE, PMD, or PUD sized page (but not compound) to be inserted by vmf_insert_mixed() which will call either insert_pfn() or insert_page(). Neither insert_pfn() nor insert_page() increments the page reference count.
But why was this done? It seems very strange to put a pfn with a struct page into a VMA and then deliberately not take the refcount for the duration of that pfn being in the VMA?
What prevents memunmap_pages() from progressing while VMAs still point at the memory?
Agreed. Adding Roger who added MEMORY_DEVICE_GENERIC and the only user.
I think just leaving the page reference count at one is better than trying to use the mmu_interval_notifier or changing vmf_insert_mixed() and invalidations of pfn_t_devmap(pfn) to adjust the page reference count.
Why so? The entire point of getting struct page's for this stuff was to be able to follow the struct page flow. I never did learn a reason why there is devmap stuff all over the place in the page table code...
Exactly.