hostmem: Wire up RAM_NORESERVE via "reserve" property
Let's provide a way to control the use of RAM_NORESERVE via memory
backends using the "reserve" property which defaults to true (old
behavior).
Only Linux currently supports clearing the flag (and support is checked at
runtime, depending on the setting of "/proc/sys/vm/overcommit_memory").
Windows and other POSIX systems will bail out with "reserve=false".
The target use case is virtio-mem, which dynamically exposes memory
inside a large, sparse memory area to the VM. This essentially allows
avoiding to set "/proc/sys/vm/overcommit_memory == 0") when using
virtio-mem and also supporting hugetlbfs in the future.
As really only Linux implements RAM_NORESERVE right now, let's expose
the property only with CONFIG_LINUX. Setting the property to "false"
will then only fail in corner cases -- for example on very old kernels
or when memory overcommit was completely disabled by the admin.
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-11-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 9b1b9f0..cd03802 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -39,6 +39,7 @@
object_get_typename(OBJECT(backend)));
#else
HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(backend);
+ uint32_t ram_flags;
gchar *name;
if (!backend->size) {
@@ -51,11 +52,11 @@
}
name = host_memory_backend_get_name(backend);
- memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
- name,
- backend->size, fb->align,
- (backend->share ? RAM_SHARED : 0) |
- (fb->is_pmem ? RAM_PMEM : 0),
+ ram_flags = backend->share ? RAM_SHARED : 0;
+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+ ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
+ memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), name,
+ backend->size, fb->align, ram_flags,
fb->mem_path, fb->readonly, errp);
g_free(name);
#endif
diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
index 3076da1..3fc85c3 100644
--- a/backends/hostmem-memfd.c
+++ b/backends/hostmem-memfd.c
@@ -54,6 +54,7 @@
name = host_memory_backend_get_name(backend);
ram_flags = backend->share ? RAM_SHARED : 0;
+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend), name,
backend->size, ram_flags, fd, 0, errp);
g_free(name);
diff --git a/backends/hostmem-ram.c b/backends/hostmem-ram.c
index 741e701..b8e55cd 100644
--- a/backends/hostmem-ram.c
+++ b/backends/hostmem-ram.c
@@ -29,6 +29,7 @@
name = host_memory_backend_get_name(backend);
ram_flags = backend->share ? RAM_SHARED : 0;
+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
memory_region_init_ram_flags_nomigrate(&backend->mr, OBJECT(backend), name,
backend->size, ram_flags, errp);
g_free(name);
diff --git a/backends/hostmem.c b/backends/hostmem.c
index aab3de8..4c05862 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -216,6 +216,11 @@
Error *local_err = NULL;
HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+ if (!backend->reserve && value) {
+ error_setg(errp, "'prealloc=on' and 'reserve=off' are incompatible");
+ return;
+ }
+
if (!host_memory_backend_mr_inited(backend)) {
backend->prealloc = value;
return;
@@ -267,6 +272,7 @@
/* TODO: convert access to globals to compat properties */
backend->merge = machine_mem_merge(machine);
backend->dump = machine_dump_guest_core(machine);
+ backend->reserve = true;
backend->prealloc_threads = 1;
}
@@ -425,6 +431,30 @@
backend->share = value;
}
+#ifdef CONFIG_LINUX
+static bool host_memory_backend_get_reserve(Object *o, Error **errp)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(o);
+
+ return backend->reserve;
+}
+
+static void host_memory_backend_set_reserve(Object *o, bool value, Error **errp)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(o);
+
+ if (host_memory_backend_mr_inited(backend)) {
+ error_setg(errp, "cannot change property value");
+ return;
+ }
+ if (backend->prealloc && !value) {
+ error_setg(errp, "'prealloc=on' and 'reserve=off' are incompatible");
+ return;
+ }
+ backend->reserve = value;
+}
+#endif /* CONFIG_LINUX */
+
static bool
host_memory_backend_get_use_canonical_path(Object *obj, Error **errp)
{
@@ -493,6 +523,12 @@
host_memory_backend_get_share, host_memory_backend_set_share);
object_class_property_set_description(oc, "share",
"Mark the memory as private to QEMU or shared");
+#ifdef CONFIG_LINUX
+ object_class_property_add_bool(oc, "reserve",
+ host_memory_backend_get_reserve, host_memory_backend_set_reserve);
+ object_class_property_set_description(oc, "reserve",
+ "Reserve swap space (or huge pages) if applicable");
+#endif /* CONFIG_LINUX */
/*
* Do not delete/rename option. This option must be considered stable
* (as if it didn't have the 'x-' prefix including deprecation period) as
