On 5/10/19 9:16 AM, Larry Bassel wrote: > On 09 May 19 09:49, Matthew Wilcox wrote: >> On Thu, May 09, 2019 at 09:05:33AM -0700, Larry Bassel wrote: >>> This is based on (but somewhat different from) what hugetlbfs >>> does to share/unshare page tables. >> >> Wow, that worked out far more cleanly than I was expecting to see. > > Yes, I was pleasantly surprised. As I've mentioned already, I > think this is at least partially due to the nature of DAX. I have not looked in detail to make sure this is indeed all the places you need to hook and special case for sharing/unsharing. Since this scheme is somewhat like that used for hugetlb, I just wanted to point out some nasty bugs related to hugetlb PMD sharing that were fixed last year. 5e41540c8a0f hugetlbfs: fix kernel BUG at fs/hugetlbfs/inode.c:444! dff11abe280b hugetlb: take PMD sharing into account when flushing tlb/caches 017b1660df89 mm: migration: fix migration of huge PMD shared pages The common issue in these is that when unmapping a page with a shared PMD mapping you need to flush the entire shared range and not just the unmapped page. The above changes were hugetlb specific. I do not know if any of this applies in the case of DAX. -- Mike Kravetz
