Linux: (hack) work around the fact that Zswapped pages may be SwapCached

linux/Platform.c

@@ -389,7 +389,22 @@ void Platform_setSwapValues(Meter* this) {
  this->values[SWAP_METER_FRONTSWAP] = 0; /* frontswap -- memory that is accounted to swap but resides elsewhere */
 
  if (lpl->zswap.usedZswapOrig > 0 || lpl->zswap.usedZswapComp > 0) {
+ /*
+ * FIXME: Zswapped pages can be both SwapUsed and SwapCached, and we do not know which.
+ *
+ * Apparently, it is possible that Zswapped > SwapUsed. This means that some of Zswapped pages
+ * were actually SwapCached, nor SwapUsed. Unfortunately, we cannot tell what exactly portion
+ * of Zswapped pages were SwapCached.
+ *
+ * For now, subtract Zswapped from SwapUsed and only if Zswapped > SwapUsed, subtract the
+ * overflow from SwapCached.
+ */
  this->values[SWAP_METER_USED] -= lpl->zswap.usedZswapOrig;
+ if (this->values[SWAP_METER_USED] < 0) {
+ /* subtract the overflow from SwapCached */
+ this->values[SWAP_METER_CACHE] += this->values[SWAP_METER_USED];
+ this->values[SWAP_METER_USED] = 0;
+ }
  this->values[SWAP_METER_FRONTSWAP] += lpl->zswap.usedZswapOrig;
  }
 }