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>

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);