read-cache: allow to keep mmap'd memory after reading

Later, we will introduce git index-helper to share this memory with
other git processes.

We only unmap it when we discard the index (although the kernel may of
course choose to page it out).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 15 insertions, 1 deletions

diff --git a/cache.h b/cache.h
index b829410f6d..4180e2b703 100644
--- a/cache.h
+++ b/cache.h
@@ -333,11 +333,14 @@ struct index_state {
 	struct split_index *split_index;
 	struct cache_time timestamp;
 	unsigned name_hash_initialized : 1,
+		 keep_mmap : 1,
 		 initialized : 1;
 	struct hashmap name_hash;
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
 	struct untracked_cache *untracked;
+	void *mmap;
+	size_t mmap_size;
 };
 
 extern struct index_state the_index;
diff --git a/read-cache.c b/read-cache.c
index 16cc487699..3cb0ec3604 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1574,6 +1574,10 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
 	if (mmap == MAP_FAILED)
 		die_errno("unable to map index file");
+	if (istate->keep_mmap) {
+		istate->mmap = mmap;
+		istate->mmap_size = mmap_size;
+	}
 	close(fd);
 
 	hdr = mmap;
@@ -1626,11 +1630,13 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 		src_offset += 8;
 		src_offset += extsize;
 	}
-	munmap(mmap, mmap_size);
+	if (!istate->keep_mmap)
+		munmap(mmap, mmap_size);
 	return istate->cache_nr;
 
 unmap:
 	munmap(mmap, mmap_size);
+	istate->mmap = NULL;
 	die("index file corrupt");
 }
 
@@ -1655,6 +1661,7 @@ int read_index_from(struct index_state *istate, const char *path)
 		discard_index(split_index->base);
 	else
 		split_index->base = xcalloc(1, sizeof(*split_index->base));
+	split_index->base->keep_mmap = istate->keep_mmap;
 	ret = do_read_index(split_index->base,
 			    git_path("sharedindex.%s",
 				     sha1_to_hex(split_index->base_sha1)), 1);
@@ -1698,6 +1705,10 @@ int discard_index(struct index_state *istate)
 	free(istate->cache);
 	istate->cache = NULL;
 	istate->cache_alloc = 0;
+	if (istate->keep_mmap && istate->mmap) {
+		munmap(istate->mmap, istate->mmap_size);
+		istate->mmap = NULL;
+	}
 	discard_split_index(istate);
 	free_untracked_cache(istate->untracked);
 	istate->untracked = NULL;