[PATCH] PM: hibernate: clean up sync_read handling in snapshot_write_next

[Brian Geffon]

In snapshot_write_next sync_read is set and unset in three different
spots unnecessiarly. As a result there is a subtle bug where the first
page after the meta data has been loaded unconditionally sets sync_read
to 0. If this first pfn was actually a highmem page then the returned
buffer will be the global "buffer," and the page needs to be loaded
synchronously.

That is, I'm not sure we can always assume the following to be safe:
		handle->buffer = get_buffer(&orig_bm, &ca);
		handle->sync_read = 0;

Because get_buffer can call get_highmem_page_buffer which can
return 'buffer'

The easiest way to address this is just set sync_read before
snapshot_write_next returns if handle->buffer == buffer.

Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
Fixes: 8357376d3df2 ("[PATCH] swsusp: Improve handling of highmem")
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/power/snapshot.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
index 190ed707ddcc..362e6bae5891 100644
--- a/kernel/power/snapshot.c
+++ b/kernel/power/snapshot.c
@@ -2780,8 +2780,6 @@ int snapshot_write_next(struct snapshot_handle *handle)
 	if (handle->cur > 1 && handle->cur > nr_meta_pages + nr_copy_pages + nr_zero_pages)
 		return 0;
 
-	handle->sync_read = 1;
-
 	if (!handle->cur) {
 		if (!buffer)
 			/* This makes the buffer be freed by swsusp_free() */
@@ -2824,7 +2822,6 @@ int snapshot_write_next(struct snapshot_handle *handle)
 			memory_bm_position_reset(&zero_bm);
 			restore_pblist = NULL;
 			handle->buffer = get_buffer(&orig_bm, &ca);
-			handle->sync_read = 0;
 			if (IS_ERR(handle->buffer))
 				return PTR_ERR(handle->buffer);
 		}
@@ -2834,9 +2831,8 @@ int snapshot_write_next(struct snapshot_handle *handle)
 		handle->buffer = get_buffer(&orig_bm, &ca);
 		if (IS_ERR(handle->buffer))
 			return PTR_ERR(handle->buffer);
-		if (handle->buffer != buffer)
-			handle->sync_read = 0;
 	}
+	handle->sync_read = (handle->buffer == buffer);
 	handle->cur++;
 
 	/* Zero pages were not included in the image, memset it and move on. */
-- 
2.42.0.515.g380fc7ccd1-goog