zram: clear IDLE flag after recompression

Patch series "zram: IDLE flag handling fixes", v2.

zram can wrongly preserve ZRAM_IDLE flag on its entries which can result
in premature post-processing (writeback and recompression) of such
entries.

This patch (of 2)

Recompression should clear ZRAM_IDLE flag on the entries 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.

The bug was reported by Shin Kawamura.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 84b33bf ("zram: introduce recompress sysfs knob")
Signed-off-by: Sergey Senozhatsky <[email protected]>
Reported-by: Shin Kawamura <[email protected]>
Acked-by: Brian Geffon <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: <[email protected]>

Author: sergey-senozhatsky

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)