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Re: [Qemu-trivial] [Qemu-devel] [PATCH v4] snapshot: use local variable


From: Max Reitz
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH v4] snapshot: use local variable to bdrv_pwrite_sync L1 table
Date: Wed, 22 Oct 2014 14:02:45 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 2014-10-22 at 14:01, Max Reitz wrote:
On 2014-10-22 at 13:59, Gonglei wrote:
On 2014/10/22 19:45, 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>
Reviewed-by: Max Reitz <address@hidden>
---
v3 -> v4:
  - convert local L1 table to host-style before copy it
    back to s->l1_table

v2 -> v3:
  - replace g_try_malloc0 with qemu_try_blockalign
  - copy the latest local L1 table back to s->l1_table
    after successfully bdrv_pwrite_sync L1 table

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 | 28 ++++++++++++----------------
  1 file changed, 12 insertions(+), 16 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2bcaaf9..3e4050a 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 = qemu_try_blockalign(bs->file, l1_size2);
+    if (l1_size2 && l1_table == NULL) {
I think this check has a logic problem. If l1_size2 != 0 and l1_table == NULL,
What will happen?

Then this condition is met and you return with -ENOMEM...?

Oh, but I see something different: qemu_try_blockalign() never returns NULL, not even when you request 0 bytes. Therefore, just "if (l1_table == NULL)" is sufficient.

Max



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