rst: Introduce memory allocator for rst blob

The memory in question is allocated and then remaped into
restorer with one call to mremap. Thus, to use the mem we
need to provide to users API for

* alloc memory
* get current allocation pointer
* convert current pointer into remapped one
* the remap itself

This would help combinding lots of small allocations such
as tcp sockets to unrepair, siginfos and posix timers.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -1370,6 +1370,103 @@ int cr_restore_tasks(void)
 	return restore_root_task(root_item);
 }
 
+/*
+ * Memory allocation for restorer blob.
+ *
+ * The mem should be visible from both contexts -- restorer
+ * and crtools (current).
+ *
+ * Memory is allocated as linear array of objects, that cannot
+ * be freed. This makes things very simple.
+ *
+ * After everything is allocated memory is mremap-ed into the
+ * new position (see comment near call to restorer_get_vma_hint)
+ */
+
+/*
+ * rst_mem -- pointer to the whole buffer
+ * rst_mem_c -- pointer to current free space
+ * rst_mem_r -- same as rst_mem, but after mremap
+ */
+
+static void *rst_mem, *rst_mem_c, *rst_mem_r;
+static unsigned long rst_mem_len;
+
+#define RST_MEM_BATCH	(2 * PAGE_SIZE)
+
+static int rst_mem_init(void)
+{
+	rst_mem = mmap(NULL, RST_MEM_BATCH, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANON, 0, 0);
+	if (rst_mem == MAP_FAILED) {
+		pr_perror("Can't create rst mem");
+		return -1;
+	}
+
+	rst_mem_c = rst_mem;
+	rst_mem_len = RST_MEM_BATCH;
+
+	return 0;
+}
+
+static void *rst_mem_alloc(unsigned long size)
+{
+	void *aux;
+
+	if (rst_mem_c + size > rst_mem + rst_mem_len) {
+		aux = mremap(rst_mem, rst_mem_len,
+				rst_mem_len + RST_MEM_BATCH, MREMAP_MAYMOVE);
+		if (aux == MAP_FAILED) {
+			pr_perror("Can't grow rst mem");
+			return NULL;
+		}
+
+		rst_mem_c += (aux - rst_mem);
+		rst_mem = aux;
+		rst_mem_len += RST_MEM_BATCH;
+	}
+
+	aux = rst_mem_c;
+	rst_mem_c += size;
+
+	return aux;
+}
+
+static int rst_mem_remap(void *to)
+{
+	if (!rst_mem_len)
+		return 0;
+
+	pr_info("Remap %p/%lu rstmem into %p\n", rst_mem, rst_mem_len, to);
+
+	if (mremap(rst_mem, rst_mem_len, rst_mem_len,
+				MREMAP_FIXED | MREMAP_MAYMOVE, to) != to) {
+		pr_perror("Can't move restorer mem");
+		return -1;
+	}
+
+	rst_mem_r = to;
+	return 0;
+}
+
+/* Current position (not addrees, as it may change after eal _alloc) */
+static inline unsigned long rst_mem_cpos(void)
+{
+	return rst_mem_c - rst_mem;
+}
+
+/* Address as seen before remap */
+static inline void *rst_mem_caddr(unsigned long pos)
+{
+	return rst_mem + pos;
+}
+
+/* Address as seen after remap */
+static inline void *rst_mem_raddr(unsigned long pos)
+{
+	return rst_mem_r + pos;
+}
+
 static long restorer_get_vma_hint(pid_t pid, struct list_head *tgt_vma_list,
 		struct list_head *self_vma_list, long vma_len)
 {
@@ -1944,6 +2041,9 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 
 	pr_info("Restore via sigreturn\n");
 
+	if (rst_mem_init())
+		goto err;
+
 	ret = parse_smaps(pid, &self_vmas, false);
 	close_proc();
 	if (ret < 0)
@@ -2006,7 +2106,8 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 				self_vmas_len + vmas_len +
 				rst_tcp_socks_size +
 				siginfo_size +
-				posix_timers_size;
+				posix_timers_size +
+				rst_mem_len;
 
 	/*
 	 * Figure out how much memory runtime vdso will need.
@@ -2138,6 +2239,10 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 	task_args->rst_tcp_socks = mem;
 	task_args->rst_tcp_socks_size = rst_tcp_socks_size;
 
+	mem += rst_tcp_socks_size;
+	if (rst_mem_remap(mem))
+		goto err;
+
 	/*
 	 * Arguments for task restoration.
 	 */
@@ -2230,7 +2335,7 @@ static int sigreturn_restore(pid_t pid, CoreEntry *core)
 	 * since we need it being accessible even when own
 	 * self-vmas are unmaped.
 	 */
-	mem += (unsigned long)rst_tcp_socks_size;
+	mem += rst_mem_len;
 	task_args->vdso_rt_parked_at = (unsigned long)mem + vdso_rt_delta;
 	task_args->vdso_sym_rt = vdso_sym_rt;