[PATCH v3] vmpressure: consider "scanned < reclaimed" case when calculating a pressure level.

[Hyunhee Kim <hyunhee.kim@samsung.com>]

In vmpressure, the pressure level is calculated based on the ratio
of how many pages were scanned vs. reclaimed in a given time window.
However, there is a possibility that "scanned < reclaimed" for some
reasons, e.g., when reclaiming ends by fatal signal in shrink_inactive_list
or THP reclaiming, etc. When this happens, we cannot tell anything about the
current pressure level. So, with this patch, we just return "low" level
when "scanned < reclaimed" happens to inform that there is reclaiming activity.
Userland can have a chance to free some memory.

Signed-off-by: Hyunhee Kim <hyunhee.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
---
 mm/vmpressure.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 736a601..915a608 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -119,6 +119,16 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 	unsigned long pressure;
 
 	/*
+	 * This could happen for some reasons. e.g., reclaiming ends by fatal
+	 * signal in shrink_inactive_list() or THP reclaiming, etc. In this case,
+	 * we cannot tell anything about the pressure level. So, the best way to
+	 * handle this is to notify LOW in order to inform that there is
+	 * reclaiming activity. This gives a chance to userland to free memory.
+	 */
+	if (reclaimed > scanned)
+		return VMPRESSURE_LOW;
+
+	/*
 	 * We calculate the ratio (in percents) of how many pages were
 	 * scanned vs. reclaimed in a given time frame (window). Note that
 	 * time is in VM reclaimer's "ticks", i.e. number of pages
-- 
1.7.9.5


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