CrOS: Fix dividing by zero in CalculateAvailableMemoryUserSpaceKB

When the ram_vs_swap_weight file is not available, the ram_swap_weight
varialbe is zero. CalculateAvailableMemoryUserSpaceKB uses the
ram_swap_weight variable as denominator and crashes. Always set
ram_swap_weight to non-zero value to avoid the dividing by zero issue.

Bug: b/171033232
Change-Id: I82e2cfd28ea21e57f3e6dd40f8cd68ee9f265c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503914
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821714}

chromeos/memory/pressure/pressure.cc

@@ -24,6 +24,9 @@ constexpr char kMinFilelist[] = "/proc/sys/vm/min_filelist_kbytes";
 constexpr char kRamVsSwapWeight[] =
     "/sys/kernel/mm/chromeos-low_mem/ram_vs_swap_weight";
 
+// The default value if the ram_vs_swap_weight file is not available.
+constexpr uint64_t kRamVsSwapWeightDefault = 4;
+
 // The extra free to trigger kernel memory reclaim earlier.
 constexpr char kExtraFree[] = "/proc/sys/vm/extra_free_kbytes";
 
@@ -233,6 +236,8 @@ void UpdateMemoryParameters() {
   reserved_free = GetReservedMemoryKB();
   min_filelist = ReadFileToUint64(base::FilePath(kMinFilelist));
   ram_swap_weight = ReadFileToUint64(base::FilePath(kRamVsSwapWeight));
+  if (ram_swap_weight == 0)
+    ram_swap_weight = kRamVsSwapWeightDefault;
 }
 
 }  // namespace pressure