mm, dax: fix livelock, allow dax pmd mappings to become writeable

Prior to this change DAX PMD mappings that were made read-only were
never able to be made writable again.  This is because the code in
insert_pfn_pmd() that calls pmd_mkdirty() and pmd_mkwrite() would skip
these calls if the PMD already existed in the page table.

Instead, if we are doing a write always mark the PMD entry as dirty and
writeable.  Without this code we can get into a condition where we mark
the PMD as read-only, and then on a subsequent write fault we get into
an infinite loop of PMD faults where we try unsuccessfully to make the
PMD writeable.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Jeff Moyer <jmoyer@redhat.com>
Reported-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9d12d63a0ddddc9c9d52fb11643f31a5252d18ea..996e86dbeb435aefc2d88e215d5ebfde8cb86e9f 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -938,15 +938,13 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
        spinlock_t *ptl;
 
        ptl = pmd_lock(mm, pmd);
-       if (pmd_none(*pmd)) {
-               entry = pmd_mkhuge(pfn_pmd(pfn, prot));
-               if (write) {
-                       entry = pmd_mkyoung(pmd_mkdirty(entry));
-                       entry = maybe_pmd_mkwrite(entry, vma);
-               }
-               set_pmd_at(mm, addr, pmd, entry);
-               update_mmu_cache_pmd(vma, addr, pmd);
+       entry = pmd_mkhuge(pfn_pmd(pfn, prot));
+       if (write) {
+               entry = pmd_mkyoung(pmd_mkdirty(entry));
+               entry = maybe_pmd_mkwrite(entry, vma);
        }
+       set_pmd_at(mm, addr, pmd, entry);
+       update_mmu_cache_pmd(vma, addr, pmd);
        spin_unlock(ptl);
 }