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Re: [Qemu-trivial] [Qemu-devel] [PATCH] snapshot: use local variable to


From: Max Reitz
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] snapshot: use local variable to bdrv_pwrite_sync L1 table
Date: Tue, 21 Oct 2014 11:24:46 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 2014-10-21 at 10:04, 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 <address@hidden>
---
  block/qcow2-refcount.c | 22 +++++++---------------
  1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2bcaaf9..8b318e8 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -881,7 +881,6 @@ 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;
@@ -889,6 +888,11 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
      l2_table = NULL;
      l1_table = NULL;

Please remove this assignment; thanks to this hunk we don't need it anymore.

      l1_size2 = l1_size * sizeof(uint64_t);
+    l1_table = g_try_malloc0(align_offset(l1_size2, 512));

I wanted to propose using qemu_try_blockalign(), but since it'd require a memset() afterwards, it gets rather ugly.

Could you at least replace 512 by BDRV_SECTOR_SIZE, and maybe even align_offset() by ROUND_UP()? We should probably do the latter in all of the qcow2 code, though, I think it's just there because it has been around since before there was a ROUND_UP()...

+    if (l1_size2 && l1_table == NULL) {
+        ret = -ENOMEM;
+        goto fail;
+    }
s->cache_discards = true; @@ -896,13 +900,6 @@ 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 +909,7 @@ 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++) {
@@ -1055,12 +1051,8 @@ fail:

I don't think it will change a lot, but could you wrap the "s->cache_discards = false; qcow2_process_discards(bs, ret);" in an "if (s->cache_discards)"? You have introduced a case where s->cache_discards was still false, so we don't need to call qcow2_process_discards() then (which will hopefully return immediately, but well...).

          }
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]);
-        }
      }
-    if (l1_allocated)
+    if (l1_table)
          g_free(l1_table);

Just drop the condition. g_free(l1_table); is enough.

      return ret;
  }

The change itself is good, it just needs some polishing.

Max



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