mem: Align mmap offset to page boundary

If we create abstract memories with a sub-page size on a system with
shared backstore, the offset of next mmap might become non-page-align
and cause an invalid argument error.

In this CL, we always upscale the range size to multiple of page before
updating the offset, so the offset is always on page boundary.

Change-Id: I3a6adf312f2cb5a09ee6a24a87adc62b630eac66
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58289
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Boris Shingarov <shingarov@labware.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/physical.cc

@@ -50,6 +50,7 @@
 #include <iostream>
 #include <string>
 
+#include "base/intmath.hh"
 #include "base/trace.hh"
 #include "debug/AddrRanges.hh"
 #include "debug/Checkpoint.hh"
@@ -81,7 +82,8 @@ PhysicalMemory::PhysicalMemory(const std::string& _name,
                                const std::string& shared_backstore,
                                bool auto_unlink_shared_backstore) :
     _name(_name), size(0), mmapUsingNoReserve(mmap_using_noreserve),
-    sharedBackstore(shared_backstore), sharedBackstoreSize(0)
+    sharedBackstore(shared_backstore), sharedBackstoreSize(0),
+    pageSize(sysconf(_SC_PAGE_SIZE))
 {
     // Register cleanup callback if requested.
     if (auto_unlink_shared_backstore && !sharedBackstore.empty()) {
@@ -217,7 +219,9 @@ PhysicalMemory::createBackingStore(
     } else {
         // Newly create backstore will be located after previous one.
         map_offset = sharedBackstoreSize;
-        sharedBackstoreSize += range.size();
+        // mmap requires the offset to be multiple of page, so we need to
+        // upscale the range size.
+        sharedBackstoreSize += roundUp(range.size(), pageSize);
         DPRINTF(AddrRanges, "Sharing backing store as %s at offset %llu\n",
                 sharedBackstore.c_str(), (uint64_t)map_offset);
         shm_fd = shm_open(sharedBackstore.c_str(), O_CREAT | O_RDWR, 0666);

src/mem/physical.hh

@@ -156,6 +156,8 @@ class PhysicalMemory : public Serializable
     const std::string sharedBackstore;
     uint64_t sharedBackstoreSize;
 
+    long pageSize;
+
     // The physical memory used to provide the memory in the simulated
     // system
     std::vector<BackingStoreEntry> backingStore;