Add thrashing page cache and direct compact related descriptions and
update the usage of getdelays userspace utility.

The following patches modifications have been updated:
https://lore.kernel.org/all/20190312102002.31737-4-jinpuwang@gmail.com/
https://lore.kernel.org/all/1638619795-71451-1-git-send-email-
wang.yong12@zte.com.cn/

Link: https://lkml.kernel.org/r/1639583021-92977-1-git-send-email-wang.yong12@zte.com.cnSigned-off-by: wangyong <wang.yong12@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>