[RFC PATCH 2/5] hugetlb: enhance hugetlb fault processing to support soft dirty

[Mike Kravetz]

hugetlb fault processing code would COW all write faults where the
pte was not writable.  Soft dirty will write protect ptes as part
of it's tracking mechanism.  The existing hugetlb_cow  code will do
the right thing for PRIVATE mappings as it checks map_count.  However,
for SHARED mappings it would actually allocate and install a COW page.
Modify the code to not call hugetlb_cow for SHARED mappings and just
update the pte.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 47f3123afd1a..b561b6867ec1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4584,8 +4584,10 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * spinlock. For private mappings, we also lookup the pagecache
 	 * page now as it is used to determine if a reservation has been
 	 * consumed.
+	 * Only non-shared mappings are sent to hugetlb_cow.
 	 */
-	if ((flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
+	if ((flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry) &&
+					!(vma->vm_flags & VM_SHARED)) {
 		if (vma_needs_reservation(h, vma, haddr) < 0) {
 			ret = VM_FAULT_OOM;
 			goto out_mutex;
@@ -4593,9 +4595,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		/* Just decrements count, does not deallocate */
 		vma_end_reservation(h, vma, haddr);
 
-		if (!(vma->vm_flags & VM_MAYSHARE))
-			pagecache_page = hugetlbfs_pagecache_page(h,
-								vma, haddr);
+		pagecache_page = hugetlbfs_pagecache_page(h, vma, haddr);
 	}
 
 	ptl = huge_pte_lock(h, mm, ptep);
@@ -4620,9 +4620,18 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (flags & FAULT_FLAG_WRITE) {
 		if (!huge_pte_write(entry)) {
-			ret = hugetlb_cow(mm, vma, address, ptep,
-					  pagecache_page, ptl);
-			goto out_put_page;
+			if (!(vma->vm_flags & VM_SHARED)) {
+				ret = hugetlb_cow(mm, vma, address, ptep,
+						pagecache_page, ptl);
+				goto out_put_page;
+			}
+
+			/* write protected for soft dirty processing */
+			if ((vma->vm_flags & VM_WRITE) &&
+					(vma->vm_flags & VM_SHARED))
+				entry = huge_pte_mkwrite(entry);
+
+			entry = huge_pte_mkdirty(entry);
 		}
 		entry = huge_pte_mkdirty(entry);
 	}
-- 
2.29.2