[PATCH] mm/pmem: Avoid inserting hugepage PTE entry with fsdax if hugepage support is disabled

[Aneesh Kumar K.V]

Differentiate between hardware not supporting hugepages and user disabling THP
via 'echo never > /sys/kernel/mm/transparent_hugepage/enabled'

For the devdax namespace, the kernel handles the above via the
supported_alignment attribute and failing to initialize the namespace
if the namespace align value is not supported on the platform.

For the fsdax namespace, the kernel will continue to initialize
the namespace. This can result in the kernel creating a huge pte
entry even though the hardware don't support the same.

We do want hugepage support with pmem even if the end-user disabled THP
via sysfs file (/sys/kernel/mm/transparent_hugepage/enabled). Hence
differentiate between hardware/firmware lacking support vs user-controlled
disable of THP and prevent a huge fault if the hardware lacks hugepage
support.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
---
 include/linux/huge_mm.h | 15 +++++++++------
 mm/huge_memory.c        |  6 +++++-
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 6a19f35f836b..ba973efcd369 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -78,6 +78,7 @@ static inline vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn,
 }
 
 enum transparent_hugepage_flag {
+	TRANSPARENT_HUGEPAGE_NEVER_DAX,
 	TRANSPARENT_HUGEPAGE_FLAG,
 	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
 	TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
@@ -123,6 +124,13 @@ extern unsigned long transparent_hugepage_flags;
  */
 static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
 {
+
+	/*
+	 * If the hardware/firmware marked hugepage support disabled.
+	 */
+	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_NEVER_DAX))
+		return false;
+
 	if (vma->vm_flags & VM_NOHUGEPAGE)
 		return false;
 
@@ -134,12 +142,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
 
 	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_FLAG))
 		return true;
-	/*
-	 * For dax vmas, try to always use hugepage mappings. If the kernel does
-	 * not support hugepages, fsdax mappings will fallback to PAGE_SIZE
-	 * mappings, and device-dax namespaces, that try to guarantee a given
-	 * mapping size, will fail to enable
-	 */
+
 	if (vma_is_dax(vma))
 		return true;
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9237976abe72..d698b7e27447 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -386,7 +386,11 @@ static int __init hugepage_init(void)
 	struct kobject *hugepage_kobj;
 
 	if (!has_transparent_hugepage()) {
-		transparent_hugepage_flags = 0;
+		/*
+		 * Hardware doesn't support hugepages, hence disable
+		 * DAX PMD support.
+		 */
+		transparent_hugepage_flags = 1 << TRANSPARENT_HUGEPAGE_NEVER_DAX;
 		return -EINVAL;
 	}
 
-- 
2.29.2