mm/highmem.c: fix zero_user_segments() with start > end

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 the zero_user_segments() implementation for for HIGHMEM +
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() unnecessarily while start == end == 0.

Link: https://lkml.kernel.org/r/87v9ab60r4.fsf@mail.parknet.co.jp
Fixes: 0060ef3b4e6d ("mm: support THPs in zero_user_segments")
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/highmem.c b/mm/highmem.c
index 874b732b120ce2c81945c8582971f645fba5a9f8..86f2b9495f9cfd3f30127e8a452f6c1e182511d9 100644 (file)
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -368,20 +368,24 @@ void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
 
        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 *page, unsigned start1, unsigned end1,
                } 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;
                }