[PATCH v3] mm: Fix mremap not considering huge pmd devmap

[Ajay Kaher]

Hi Fan,

Able to reproduce this issue on v4.19.y using your test program.

And as per commit message it fixes commit 5c7fb56e5e3f 
("mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmâ) at kernel version v4.5.
So, v4.9.y should be vulnerable, however not able to reproduce on v4.9.y.
Does any specific scenerio need to test for v4.9.y?

For v4.9, modified test program as MAP_SHARED_VALIDATE is not available:
- return mmap(NULL, REGION_PM_SIZE, PROT, MAP_SHARED_VALIDATE|MAP_SYNC,
+ return mmap(NULL, REGION_PM_SIZE, PROT, MAP_SHARED|MAP_SYNC,

Let me know if I need to test some other way for v4.9.y.

-Ajay