thodg/libgit2

diff --git a/src/indexer.c b/src/indexer.c
index 9aa0925..1ffbc27 100644
--- a/src/indexer.c
+++ b/src/indexer.c
@@ -449,7 +449,7 @@ static void hash_partially(git_indexer *idx, const uint8_t *data, size_t size)
 static int write_at(git_indexer *idx, const void *data, git_off_t offset, size_t size)
 {
 	git_file fd = idx->pack->mwf.fd;
-	size_t page_size;
+	size_t mmap_alignment;
 	size_t page_offset;
 	git_off_t page_start;
 	unsigned char *map_data;
@@ -458,11 +458,11 @@ static int write_at(git_indexer *idx, const void *data, git_off_t offset, size_t
 
 	assert(data && size);
 
-	if ((error = git__page_size(&page_size)) < 0)
+	if ((error = git__mmap_alignment(&mmap_alignment)) < 0)
 		return error;
 
-	/* the offset needs to be at the beginning of the a page boundary */
-	page_offset = offset % page_size;
+	/* the offset needs to be at the mmap boundary for the platform */
+	page_offset = offset % mmap_alignment;
 	page_start = offset - page_offset;
 
 	if ((error = p_mmap(&map, page_offset + size, GIT_PROT_WRITE, GIT_MAP_SHARED, fd, page_start)) < 0)
diff --git a/src/posix.c b/src/posix.c
index c7201ba..b3f1a1c 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -224,6 +224,13 @@ int git__page_size(size_t *page_size)
 	return 0;
 }
 
+int git__mmap_alignment(size_t *alignment)
+{
+	/* dummy; here we don't need any alignment anyway */
+	*alignment = 4096;
+	return 0;
+}
+
 
 int p_mmap(git_map *out, size_t len, int prot, int flags, int fd, git_off_t offset)
 {
diff --git a/src/posix.h b/src/posix.h
index 8785a4c..f204751 100644
--- a/src/posix.h
+++ b/src/posix.h
@@ -109,6 +109,7 @@ extern int p_getcwd(char *buffer_out, size_t size);
 extern int p_rename(const char *from, const char *to);
 
 extern int git__page_size(size_t *page_size);
+extern int git__mmap_alignment(size_t *page_size);
 
 /**
  * Platform-dependent methods
diff --git a/src/unix/map.c b/src/unix/map.c
index 72abb34..c55ad1a 100644
--- a/src/unix/map.c
+++ b/src/unix/map.c
@@ -24,6 +24,11 @@ int git__page_size(size_t *page_size)
 	return 0;
 }
 
+int git__mmap_alignment(size_t *alignment)
+{
+  return git__page_size(alignment);
+}
+
 int p_mmap(git_map *out, size_t len, int prot, int flags, int fd, git_off_t offset)
 {
 	int mprot = PROT_READ;
diff --git a/src/win32/map.c b/src/win32/map.c
index a99c30f..03a3646 100644
--- a/src/win32/map.c
+++ b/src/win32/map.c
@@ -17,22 +17,41 @@ static DWORD get_page_size(void)
 
 	if (!page_size) {
 		GetSystemInfo(&sys);
-		page_size = sys.dwAllocationGranularity;
+		page_size = sys.dwPageSize;
 	}
 
 	return page_size;
 }
 
+static DWORD get_allocation_granularity(void)
+{
+	static DWORD granularity;
+	SYSTEM_INFO sys;
+
+	if (!granularity) {
+		GetSystemInfo(&sys);
+		granularity = sys.dwAllocationGranularity;
+	}
+
+	return granularity;
+}
+
 int git__page_size(size_t *page_size)
 {
 	*page_size = get_page_size();
 	return 0;
 }
 
+int git__mmap_alignment(size_t *page_size)
+{
+	*page_size = get_allocation_granularity();
+	return 0;
+}
+
 int p_mmap(git_map *out, size_t len, int prot, int flags, int fd, git_off_t offset)
 {
 	HANDLE fh = (HANDLE)_get_osfhandle(fd);
-	DWORD page_size = get_page_size();
+	DWORD alignment = get_allocation_granularity();
 	DWORD fmap_prot = 0;
 	DWORD view_prot = 0;
 	DWORD off_low = 0;
@@ -62,12 +81,12 @@ int p_mmap(git_map *out, size_t len, int prot, int flags, int fd, git_off_t offs
 	if (prot & GIT_PROT_READ)
 		view_prot |= FILE_MAP_READ;
 
-	page_start = (offset / page_size) * page_size;
+	page_start = (offset / alignment) * alignment;
 	page_offset = offset - page_start;
 
-	if (page_offset != 0) { /* offset must be multiple of page size */
+	if (page_offset != 0) { /* offset must be multiple of the allocation granularity */
 		errno = EINVAL;
-		giterr_set(GITERR_OS, "Failed to mmap. Offset must be multiple of page size");
+		giterr_set(GITERR_OS, "Failed to mmap. Offset must be multiple of allocation granularity");
 		return -1;
 	}