Add method to get usage stats for a single map.

The new scudo allocator creates a huge map in 64 bit address space. Some
tests would get the usage stats of all maps, but only really cared about
a small set of maps. In this case, provide a method to only get the usage
stats for the maps you care about.

Test: Unit tests pass.
Change-Id: Ie845e47a8c789a57bf689c9f0e5878a921640e30
diff --git a/libmeminfo/procmeminfo.cpp b/libmeminfo/procmeminfo.cpp
index 6f68ab4..9e9a705 100644
--- a/libmeminfo/procmeminfo.cpp
+++ b/libmeminfo/procmeminfo.cpp
@@ -244,6 +244,15 @@
     return true;
 }
 
+static int GetPagemapFd(pid_t pid) {
+    std::string pagemap_file = ::android::base::StringPrintf("/proc/%d/pagemap", pid);
+    int fd = TEMP_FAILURE_RETRY(open(pagemap_file.c_str(), O_RDONLY | O_CLOEXEC));
+    if (fd == -1) {
+        PLOG(ERROR) << "Failed to open " << pagemap_file;
+    }
+    return fd;
+}
+
 bool ProcMemInfo::ReadMaps(bool get_wss, bool use_pageidle, bool get_usage_stats) {
     // Each object reads /proc/<pid>/maps only once. This is done to make sure programs that are
     // running for the lifetime of the system can recycle the objects and don't have to
@@ -269,11 +278,8 @@
         return true;
     }
 
-    std::string pagemap_file = ::android::base::StringPrintf("/proc/%d/pagemap", pid_);
-    ::android::base::unique_fd pagemap_fd(
-            TEMP_FAILURE_RETRY(open(pagemap_file.c_str(), O_RDONLY | O_CLOEXEC)));
-    if (pagemap_fd < 0) {
-        PLOG(ERROR) << "Failed to open " << pagemap_file;
+    ::android::base::unique_fd pagemap_fd(GetPagemapFd(pid_));
+    if (pagemap_fd == -1) {
         return false;
     }
 
@@ -290,6 +296,20 @@
     return true;
 }
 
+bool ProcMemInfo::FillInVmaStats(Vma& vma) {
+    ::android::base::unique_fd pagemap_fd(GetPagemapFd(pid_));
+    if (pagemap_fd == -1) {
+        return false;
+    }
+
+    if (!ReadVmaStats(pagemap_fd.get(), vma, get_wss_, false)) {
+        LOG(ERROR) << "Failed to read page map for vma " << vma.name << "[" << vma.start << "-"
+                   << vma.end << "]";
+        return false;
+    }
+    return true;
+}
+
 bool ProcMemInfo::ReadVmaStats(int pagemap_fd, Vma& vma, bool get_wss, bool use_pageidle) {
     PageAcct& pinfo = PageAcct::Instance();
     if (get_wss && use_pageidle && !pinfo.InitPageAcct(true)) {