[RFC PATCH v9 6/7] mm: Update alloc_set_pte() for zero page

[Yu-cheng Yu]

Update alloc_set_pte() for the zero-page case.  A special mapping can also
use a zero page for read.  One use case is the Indirect Branch Tracking
legacy code page (introduced next).

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
 mm/memory.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 6daa28614327..58c1e4b60991 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3447,6 +3447,12 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 
 	flush_icache_page(vma, page);
 	entry = mk_pte(page, vma->vm_page_prot);
+
+	if (is_zero_pfn(pte_pfn(entry))) {
+		entry = pte_mkspecial(entry);
+		goto alloc_set_pte_out;
+	}
+
 	if (write)
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 	/* copy-on-write page */
@@ -3459,6 +3465,8 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 		inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
 		page_add_file_rmap(page, false);
 	}
+
+alloc_set_pte_out:
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
 
 	/* no need to invalidate: a not-present page won't be cached */
-- 
2.21.0