[PATCH v2] Fix zero_user_segments() with start > end

[OGAWA Hirofumi]

zero_user_segments() is used from __block_write_begin_int(), for
example like the following

	zero_user_segments(page, 4096, 1024, 512, 918)

But new zero_user_segments() implements for HIGMEM + TRANSPARENT_HUGEPAGE 
doesn't handle "start > end" case correctly, and hits BUG_ON(). (we
can fix __block_write_begin_int() instead though, it is the old and
multiple usage)

Also it calls kmap_atomic() unnecessary while start == end == 0.

Fixes: 0060ef3b4e6d ("mm: support THPs in zero_user_segments")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
---
 mm/highmem.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/highmem.c b/mm/highmem.c
index 874b732..86f2b94 100644
--- a/mm/highmem.c	2021-02-20 12:56:49.037165666 +0900
+++ b/mm/highmem.c	2021-02-20 22:03:08.369361223 +0900
@@ -368,20 +368,24 @@ void zero_user_segments(struct page *pag
 
 	BUG_ON(end1 > page_size(page) || end2 > page_size(page));
 
+	if (start1 >= end1)
+		start1 = end1 = 0;
+	if (start2 >= end2)
+		start2 = end2 = 0;
+
 	for (i = 0; i < compound_nr(page); i++) {
 		void *kaddr = NULL;
 
-		if (start1 < PAGE_SIZE || start2 < PAGE_SIZE)
-			kaddr = kmap_atomic(page + i);
-
 		if (start1 >= PAGE_SIZE) {
 			start1 -= PAGE_SIZE;
 			end1 -= PAGE_SIZE;
 		} else {
 			unsigned this_end = min_t(unsigned, end1, PAGE_SIZE);
 
-			if (end1 > start1)
+			if (end1 > start1) {
+				kaddr = kmap_atomic(page + i);
 				memset(kaddr + start1, 0, this_end - start1);
+			}
 			end1 -= this_end;
 			start1 = 0;
 		}
@@ -392,8 +396,11 @@ void zero_user_segments(struct page *pag
 		} else {
 			unsigned this_end = min_t(unsigned, end2, PAGE_SIZE);
 
-			if (end2 > start2)
+			if (end2 > start2) {
+				if (!kaddr)
+					kaddr = kmap_atomic(page + i);
 				memset(kaddr + start2, 0, this_end - start2);
+			}
 			end2 -= this_end;
 			start2 = 0;
 		}
_
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>