Adjust arc_p based on "bytes" in arc_shrink

In an attempt to prevent arc_c from collapsing "too fast", the
arc_shrink() function was updated to take a "bytes" parameter by this
change:

    commit 302f753
    Author: Brian Behlendorf <[email protected]>
    Date:   Tue Mar 13 14:29:16 2012 -0700

        Integrate ARC more tightly with Linux

Unfortunately, that change failed to make a similar change to the way
that arc_p was updated. So, there still exists the possibility for arc_p
to collapse to near 0 when the kernel start calling the arc's shrinkers.

This change attempts to fix this, by decrementing arc_p by the "bytes"
parameter in the same way that arc_c is updated.

In addition, a minimum value of arc_p is attempted to be maintained,
similar to the way a minimum arc_p value is maintained in arc_adapt().

Signed-off-by: Prakash Surya <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#2110

module/zfs/arc.c

@@ -2332,6 +2332,7 @@ void
 arc_shrink(uint64_t bytes)
 {
 	if (arc_c > arc_c_min) {
+		uint64_t arc_p_min;
 		uint64_t to_free;
 
 		to_free = bytes ? bytes : arc_c >> zfs_arc_shrink_shift;
@@ -2341,7 +2342,14 @@ arc_shrink(uint64_t bytes)
 		else
 			arc_c = arc_c_min;
 
-		atomic_add_64(&arc_p, -(arc_p >> zfs_arc_shrink_shift));
+		arc_p_min = (arc_c >> zfs_arc_p_min_shift);
+		to_free = bytes ? bytes : arc_p >> zfs_arc_shrink_shift;
+
+		if (arc_p > arc_p_min + to_free)
+			atomic_add_64(&arc_p, -to_free);
+		else
+			arc_p = arc_p_min;
+
 		if (arc_c > arc_size)
 			arc_c = MAX(arc_size, arc_c_min);
 		if (arc_p > arc_c)