[PATCH -mm 05/25] mm, dax: fix livelock, allow dax pmd mappings to become writeable

[Dan Williams <dan.j.williams@intel.com>]

From: Ross Zwisler <ross.zwisler@linux.intel.com>

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.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Reported-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 mm/huge_memory.c |   14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b41793b12a2d..70323839bd0d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -967,15 +967,13 @@ 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);
 }
 

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