mm: allow a NULL fn callback in apply_to_page_range

Besides calling the callback on each page, apply_to_page_range also has
the effect of pre-faulting all PTEs for the range.  To support callers
that only need the pre-faulting, make the callback optional.

Based on a patch from Minchan Kim <[email protected]>.

Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 9 insertions, 7 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 589afe45d0b3..c48f8df6e502 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2391,13 +2391,15 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 
 	arch_enter_lazy_mmu_mode();
 
-	do {
-		if (create || !pte_none(*pte)) {
-			err = fn(pte++, addr, data);
-			if (err)
-				break;
-		}
-	} while (addr += PAGE_SIZE, addr != end);
+	if (fn) {
+		do {
+			if (create || !pte_none(*pte)) {
+				err = fn(pte++, addr, data);
+				if (err)
+					break;
+			}
+		} while (addr += PAGE_SIZE, addr != end);
+	}
 	*mask |= PGTBL_PTE_MODIFIED;
 
 	arch_leave_lazy_mmu_mode();