Fix arc_p aggressive increase

The original ARC paper called for an initial 50/50 MRU/MFU split
and this is accounted in various places where arc_p = arc_c >> 1,
with further adjustment based on ghost lists size/hit. However, in
current code both arc_adapt() and arc_get_data_impl() aggressively
grow arc_p until arc_c is reached, causing unneeded pressure on
MFU and greatly reducing its scan-resistance until ghost list
adjustments kick in.

This patch restores the original behavior of initially having arc_p
as 1/2 of total ARC, without preventing MRU to use up to 100% total
ARC when MFU is empty.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Gionatan Danti <[email protected]>
Closes openzfs#14137
Closes openzfs#14120

module/zfs/arc.c

@@ -5166,7 +5166,7 @@ arc_adapt(int bytes, arc_state_t *state)
 		atomic_add_64(&arc_c, (int64_t)bytes);
 		if (arc_c > arc_c_max)
 			arc_c = arc_c_max;
-		else if (state == arc_anon)
+		else if (state == arc_anon && arc_p < arc_c >> 1)
 			atomic_add_64(&arc_p, (int64_t)bytes);
 		if (arc_p > arc_c)
 			arc_p = arc_c;
@@ -5379,7 +5379,8 @@ arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag,
 		if (aggsum_upper_bound(&arc_sums.arcstat_size) < arc_c &&
 		    hdr->b_l1hdr.b_state == arc_anon &&
 		    (zfs_refcount_count(&arc_anon->arcs_size) +
-		    zfs_refcount_count(&arc_mru->arcs_size) > arc_p))
+		    zfs_refcount_count(&arc_mru->arcs_size) > arc_p &&
+		    arc_p < arc_c >> 1))
 			arc_p = MIN(arc_c, arc_p + size);
 	}
 }