Enchance the shared page chunk allocator.

Do not rely on the busy state of the page from which we allocate the
chunk, to protect allocator state. Use statically allocated sx lock
instead.

Provide more flexible KPI. In particular, allow to allocate chunk
without providing initial data, and allow writes into existing
allocation. Allow to get an sf buf which temporary maps the chunk, to
allow sequential updates to shared page content without unmapping in
between.

Reviewed by:	jhb
Tested by:	flo
MFC after:	1 month

1 files changed, 63 insertions, 14 deletions

diff --git a/sys/kern/kern_exec.c b/sys/kern/kern_exec.c
index dac4703e6afa..d9baab6eee56 100644
--- a/sys/kern/kern_exec.c
+++ b/sys/kern/kern_exec.c
@@ -1512,33 +1512,81 @@ exec_unregister(execsw_arg)
 	return (0);
 }
 
+static struct sx shared_page_alloc_sx;
 static vm_object_t shared_page_obj;
 static int shared_page_free;
 
-int
-shared_page_fill(int size, int align, const char *data)
+struct sf_buf *
+shared_page_write_start(int base)
 {
 	vm_page_t m;
 	struct sf_buf *s;
+
+	VM_OBJECT_LOCK(shared_page_obj);
+	m = vm_page_grab(shared_page_obj, OFF_TO_IDX(base), VM_ALLOC_RETRY);
+	VM_OBJECT_UNLOCK(shared_page_obj);
+	s = sf_buf_alloc(m, SFB_DEFAULT);
+	return (s);
+}
+
+void
+shared_page_write_end(struct sf_buf *sf)
+{
+	vm_page_t m;
+
+	m = sf_buf_page(sf);
+	sf_buf_free(sf);
+	VM_OBJECT_LOCK(shared_page_obj);
+	vm_page_wakeup(m);
+	VM_OBJECT_UNLOCK(shared_page_obj);
+}
+
+void
+shared_page_write(int base, int size, const void *data)
+{
+	struct sf_buf *sf;
 	vm_offset_t sk;
+
+	sf = shared_page_write_start(base);
+	sk = sf_buf_kva(sf);
+	bcopy(data, (void *)(sk + (base & PAGE_MASK)), size);
+	shared_page_write_end(sf);
+}
+
+static int
+shared_page_alloc_locked(int size, int align)
+{
 	int res;
 
-	VM_OBJECT_LOCK(shared_page_obj);
-	m = vm_page_grab(shared_page_obj, 0, VM_ALLOC_RETRY);
 	res = roundup(shared_page_free, align);
 	if (res + size >= IDX_TO_OFF(shared_page_obj->size))
 		res = -1;
-	else {
-		VM_OBJECT_UNLOCK(shared_page_obj);
-		s = sf_buf_alloc(m, SFB_DEFAULT);
-		sk = sf_buf_kva(s);
-		bcopy(data, (void *)(sk + res), size);
+	else
 		shared_page_free = res + size;
-		sf_buf_free(s);
-		VM_OBJECT_LOCK(shared_page_obj);
-	}
-	vm_page_wakeup(m);
-	VM_OBJECT_UNLOCK(shared_page_obj);
+	return (res);
+}
+
+int
+shared_page_alloc(int size, int align)
+{
+	int res;
+
+	sx_xlock(&shared_page_alloc_sx);
+	res = shared_page_alloc_locked(size, align);
+	sx_xunlock(&shared_page_alloc_sx);
+	return (res);
+}
+
+int
+shared_page_fill(int size, int align, const void *data)
+{
+	int res;
+
+	sx_xlock(&shared_page_alloc_sx);
+	res = shared_page_alloc_locked(size, align);
+	if (res != -1)
+		shared_page_write(res, size, data);
+	sx_xunlock(&shared_page_alloc_sx);
 	return (res);
 }
 
@@ -1547,6 +1595,7 @@ shared_page_init(void *dummy __unused)
 {
 	vm_page_t m;
 
+	sx_init(&shared_page_alloc_sx, "shpsx");
 	shared_page_obj = vm_pager_allocate(OBJT_PHYS, 0, PAGE_SIZE,
 	    VM_PROT_DEFAULT, 0, NULL);
 	VM_OBJECT_LOCK(shared_page_obj);