Handle EAGAIN error for the mincore syscall used

content/common/host_discardable_shared_memory_manager.cc

Index: content/common/host_discardable_shared_memory_manager.cc
diff --git a/content/common/host_discardable_shared_memory_manager.cc b/content/common/host_discardable_shared_memory_manager.cc
index 90eee02594c38a9629b84a443c3ed0076ba2c9e2..1568376993f52b0aa66275ed8fa5803648889bbf 100644
--- a/content/common/host_discardable_shared_memory_manager.cc
+++ b/content/common/host_discardable_shared_memory_manager.cc
@@ -210,16 +210,20 @@ bool HostDiscardableSharedMemoryManager::OnMemoryDump(
 #if defined(COUNT_RESIDENT_BYTES_SUPPORTED)
       if (args.level_of_detail ==
           base::trace_event::MemoryDumpLevelOfDetail::DETAILED) {
-        size_t resident_size =
-            base::trace_event::ProcessMemoryDump::CountResidentBytes(
-                segment->memory()->memory(), segment->memory()->mapped_size());
+        size_t resident_size;
 
         // This is added to the global dump since it has to be attributed to
         // both the allocator dumps involved.
-        pmd->GetSharedGlobalAllocatorDump(shared_segment_guid)
-            ->AddScalar("resident_size",
-                        base::trace_event::MemoryAllocatorDump::kUnitsBytes,
-                        static_cast<uint64>(resident_size));
+        if (base::trace_event::ProcessMemoryDump::CountResidentBytes(
+                segment->memory()->memory(), segment->memory()->mapped_size(),
+                &resident_size)) {
+          pmd->GetSharedGlobalAllocatorDump(shared_segment_guid)
+              ->AddScalar("resident_size",
+                          base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+                          static_cast<uint64>(resident_size));
+        } else {
+          NOTREACHED();

[reveman, 2015/10/14 12:31:29]

please

bool rv = base::trace_event::ProcessMemoryDump::CountResidentBytes(...
DCHECK(rv);

instead.

but if we keep having a DCHECK/NOTREACHED here, how is allowing
CountResidentBytes to fail fix anything?

[ssid, 2015/10/14 12:44:29]

In general it should not fail. What I want is in release builds it should show
right values. But in debug build it is good to see if this is failing and not
hide it in the tests running.

[reveman, 2015/10/14 13:03:53]

ok, sounds good. I don't like when NOTREACHED/DCHECKs are part of the structure
of the code. It should be possible to removed the DCHECK without making any
other changes and the code should still work and not look weird.

Can we do the following instead and dump resident_size=0 when it fails?

size_t resident_size = 0;
bool rv = base::trace_event::ProcessMemoryDump::CountResidentBytes(...
DCHECK(rv);
pmd->GetSharedGlobalAllocatorDump(shared_segment_guid)
              ->AddScalar(...

[ssid, 2015/10/14 13:35:55]

Done.

+        }
       }
 #endif  // defined(COUNT_RESIDENT_BYTES_SUPPORTED)
     }