Recompression should clear ZRAM_IDLE flag on the entires
it has accessed, because otherwise some entries, specifically
those for which recompression has failed, become immediate
candidate entries for another post-processing (e.g. writeback).

Consider the following case:
- recompression marks entries IDLE every 4 hours and attempts
  to recompress them
- some entries are incompressible, so we keep them intact and
  hence preserve IDLE flag
- writeback marks entries IDLE every 8 hours and writebacks
  IDLE entries, however we have IDLE entries left from
  recompression, so writeback prematurely writebacks those
  entries.

Reported-by: Shin Kawamura <kawasin@google.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index e6d12e81241d..764c5b79b42b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1864,6 +1864,13 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	if (ret)
 		return ret;
 
+	/*
+	* We touched this entry so mark it as non-IDLE. This makes sure that
+	* we don't preserve IDLE flag and don't incorrectly pick this entry
+	* for different post-processing type (e.g. writeback).
+	*/
+	zram_clear_flag(zram, index, ZRAM_IDLE);
+
 	class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old);
 	/*
 	 * Iterate the secondary comp algorithms list (in order of priority)
-- 
2.47.0.163.g1226f6d8fa-goog