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Re: [Qemu-devel] [PATCH v2] snapshot: use local variable to bdrv_pwrite_


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2] snapshot: use local variable to bdrv_pwrite_sync L1 table
Date: Tue, 21 Oct 2014 14:01:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 2014-10-21 at 13:34, 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>
---
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.

(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.

Sorry for having missed these in my previous review.

Max

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




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