zsmalloc: zs_object_copy: add clarifying comment
Patch series "tidy up zsmalloc implementation"
This patchset remove some unnecessary checks and adds a clarifying
comment. While analysing zs_object_copy() function code, I spent some
time to understand what the call kunmap_atomic(d_addr) is for. It seems
that this point is not trivial and it is worth adding a comment.
This patch (of 2):
It's not obvious why kunmap_atomic(d_addr) call is needed.
[akpm@linux-foundation.org: tweak comment layout]
Link: https://lkml.kernel.org/r/20220811153755.16102-1-avromanov@sberdevices.ru
Link: https://lkml.kernel.org/r/20220811153755.16102-2-avromanov@sberdevices.ru
Signed-off-by: Alexey Romanov <avromanov@sberdevices.ru>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 907c9b1..09ab91a 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1555,6 +1555,13 @@ static void zs_object_copy(struct size_class *class, unsigned long dst,
d_off += size;
d_size -= size;
+ /*
+ * Calling kunmap_atomic(d_addr) is necessary. kunmap_atomic()
+ * calls must occurs in reverse order of calls to kmap_atomic().
+ * So, to call kunmap_atomic(s_addr) we should first call
+ * kunmap_atomic(d_addr). For more details see
+ * https://lore.kernel.org/linux-mm/5512421D.4000603@samsung.com/
+ */
if (s_off >= PAGE_SIZE) {
kunmap_atomic(d_addr);
kunmap_atomic(s_addr);
