Re: [PATCH v3 3/6] mm: support THP migration to device private memory

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

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.