Re: [Qemu-devel] [Qemu-stable] [PATCH v2] snapshot: use local variable tobdrv_pwrite_sync L1 table

[Max Reitz]

On 2014-10-21 at 15:08, Zhang Haoyu wrote:
Use local variable to bdrv_pwrite_sync L1 table,
needless to make conversion of cached L1 table between
big-endian and host style.
Signed-off-by: Zhang Haoyu <zhan...@sangfor.com>
---
v1 -> v2:
   - remove the superflous assignment, l1_table = NULL;
   - replace 512 with BDRV_SECTOR_SIZE, and align_offset with ROUND_UP
   - remove needless check of if (l1_table) before g_free(l1_table)

   block/qcow2-refcount.c | 24 +++++++-----------------
   1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2bcaaf9..29a916a 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -881,14 +881,17 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
   {
       BDRVQcowState *s = bs->opaque;
       uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
-    bool l1_allocated = false;
       int64_t old_offset, old_l2_offset;
       int i, j, l1_modified = 0, nb_csectors, refcount;
       int ret;
   
       l2_table = NULL;
-    l1_table = NULL;
       l1_size2 = l1_size * sizeof(uint64_t);
+    l1_table = g_try_malloc0(ROUND_UP(l1_size2, BDRV_SECTOR_SIZE));
I'm sorry, but I just now realized that we don't even need the 0 variant
here. We are never accessing any element beyond l1_table[l1_size - 1],
and all elements from 0 until l1_size - 1 are overwritten by
bdrv_pread() or memcpy(). Therefore we can simply use
qemu_try_blockalign(bs->file, l1_size2) here.

OK, I will replace g_try_malloc0(ROUND_UP(l1_size2, BDRV_SECTOR_SIZE)) with
qemu_try_blockalign(bs, ROUND_UP(l1_size2, BDRV_SECTOR_SIZE)).

You can actually omit the ROUND_UP(). There are no accesses beyond 
l1_table[l1_size - 1], if I'm not mistaken.

Max

(and we should be using qemu_{try_,}blockalign() instead of
g_{try_,}malloc{0,}() whenever possible for performance reasons)

+    if (l1_size2 && l1_table == NULL) {
+        ret = -ENOMEM;
+        goto fail;
+    }
   
       s->cache_discards = true;
   
@@ -896,13 +899,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
        * l1_table_offset when it is the current s->l1_table_offset! Be careful
        * when changing this! */
       if (l1_table_offset != s->l1_table_offset) {
-        l1_table = g_try_malloc0(align_offset(l1_size2, 512));
-        if (l1_size2 && l1_table == NULL) {
-            ret = -ENOMEM;
-            goto fail;
-        }
-        l1_allocated = true;
-
           ret = bdrv_pread(bs->file, l1_table_offset, l1_table, l1_size2);
           if (ret < 0) {
               goto fail;
@@ -912,8 +908,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
               be64_to_cpus(&l1_table[i]);
       } else {
           assert(l1_size == s->l1_size);
-        l1_table = s->l1_table;
-        l1_allocated = false;
+        memcpy(l1_table, s->l1_table, l1_size2);
       }
   
       for(i = 0; i < l1_size; i++) {
@@ -1055,13 +1050,8 @@ fail:
           }
   
           ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2);
-
-        for (i = 0; i < l1_size; i++) {
-            be64_to_cpus(&l1_table[i]);
-        }
But I realized something even more important as well: If you are
modifying the current L1 table, you need to copy the result back to
s->l1_table.

My fault.
Sorry for having missed these in my previous review.

Max

       }
-    if (l1_allocated)
-        g_free(l1_table);
+    g_free(l1_table);
       return ret;
   }