[Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Alberto Garcia]

After having emptied the cache, the data in the cache tables is no
longer useful, so we can tell the kernel that we are done with it. In
Linux this frees the resources associated with it.

The effect of this can be seen in the HMP commit operation: it moves
data from the top to the base image (and fills both caches), then it
empties the top image. At this point the data in that cache is no
longer needed so it's just wasting memory.

Signed-off-by: Alberto Garcia <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
---
 block/qcow2-cache.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index ed92a09..ed14a92 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -22,8 +22,10 @@
  * THE SOFTWARE.
  */
 
+#include <sys/mman.h>
 #include "block/block_int.h"
 #include "qemu-common.h"
+#include "qemu/osdep.h"
 #include "qcow2.h"
 #include "trace.h"
 
@@ -60,6 +62,22 @@ static inline int qcow2_cache_get_table_idx(BlockDriverState 
*bs,
     return idx;
 }
 
+static void qcow2_cache_table_release(BlockDriverState *bs, Qcow2Cache *c,
+                                      int i, int num_tables)
+{
+#if QEMU_MADV_DONTNEED != QEMU_MADV_INVALID
+    BDRVQcowState *s = bs->opaque;
+    void *t = qcow2_cache_get_table_addr(bs, c, i);
+    long align = sysconf(_SC_PAGESIZE);
+    size_t mem_size = (size_t) s->cluster_size * num_tables;
+    size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
+    size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);
+    if (length > 0) {
+        qemu_madvise((uint8_t *) t + offset, length, QEMU_MADV_DONTNEED);
+    }
+#endif
+}
+
 Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
 {
     BDRVQcowState *s = bs->opaque;
@@ -237,6 +255,8 @@ int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c)
         c->entries[i].lru_counter = 0;
     }
 
+    qcow2_cache_table_release(bs, c, 0, c->size);
+
     c->lru_counter = 0;
 
     return 0;
-- 
2.1.4