Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

[James Houghton]

> The original approach was implemented in RFC v1, but the
> implementation was broken: the way refcount was handled was wrong; it
> was incremented once for each new page table mapping. (How?
> find_lock_page(), called once per hugetlb_no_page/UFFDIO_CONTINUE
> would increment refcount and we wouldn't drop it, and in
> __unmap_hugepage_range(), the mmu_gather bits would decrement the
> refcount once per mapping.)
>
> At the time, I figured the complexity of handling mapcount AND
> refcount correctly in the original approach would be quite complex, so
> I switched to the new one.

Sorry I didn't make this clear... the following steps are how we could
correctly implement the original approach.

> 1. In places that already change the mapcount, check that we're
> installing the hstate-level PTE, not a high-granularity PTE. Adjust
> mapcount AND refcount appropriately.
> 2. In the HGM walking bits, to the caller if we made the hstate-level
> PTE present. (hugetlb_[pmd,pte]_alloc is the source of truth.) Need to
> keep track of this until we figure out which page we're allocating
> PTEs for, then change mapcount/refcount appropriately.
> 3. In unmapping bits, change mmu_gather/tlb bits to drop refcount only
> once per hugepage. (This is probably the hardest of these three things
> to get right.)