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Re: [Qemu-devel] [PATCH v5 06/26] qcow2: Use 64 bits for refcount values
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v5 06/26] qcow2: Use 64 bits for refcount values |
Date: |
Tue, 3 Feb 2015 20:26:32 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 15.12.2014 um 13:50 hat Max Reitz geschrieben:
> Refcounts may have a width of up to 64 bits, so qemu should use the same
> width to represent refcount values internally.
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> block/qcow2-cluster.c | 2 +-
> block/qcow2-refcount.c | 46 ++++++++++++++++++++++------------------------
> block/qcow2.h | 4 ++--
> 3 files changed, 25 insertions(+), 27 deletions(-)
> @@ -897,11 +895,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> int64_t l1_table_offset, int l1_size, int addend)
> {
Your leaving addend an int here...
> BDRVQcowState *s = bs->opaque;
> - uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
> + uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2, refcount;
> bool l1_allocated = false;
> int64_t old_offset, old_l2_offset;
> int i, j, l1_modified = 0, nb_csectors;
> - uint16_t refcount;
> int ret;
>
> l2_table = NULL;
> @@ -968,7 +965,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> if (addend != 0) {
> ret = update_refcount(bs,
> (offset & s->cluster_offset_mask) & ~511,
> - nb_csectors * 512, abs(addend), addend < 0,
> + nb_csectors * 512, imaxabs(addend), addend <
> 0,
> QCOW2_DISCARD_SNAPSHOT);
> if (ret < 0) {
> goto fail;
> @@ -999,7 +996,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> }
> if (addend != 0) {
> ret = qcow2_update_cluster_refcount(bs,
> - cluster_index, abs(addend), addend < 0,
> + cluster_index, imaxabs(addend), addend <
> 0,
> QCOW2_DISCARD_SNAPSHOT);
> if (ret < 0) {
> goto fail;
> @@ -1042,7 +1039,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> if (addend != 0) {
> ret = qcow2_update_cluster_refcount(bs, l2_offset >>
> s->cluster_bits,
> - abs(addend), addend < 0,
> + imaxabs(addend), addend
> < 0,
> QCOW2_DISCARD_SNAPSHOT);
> if (ret < 0) {
> goto fail;
...but still replace abs() by imaxabs(). Did you intend to convert
addend or why this change?
> @@ -1658,7 +1655,7 @@ static void compare_refcounts(BlockDriverState *bs,
> BdrvCheckResult *res,
> {
> BDRVQcowState *s = bs->opaque;
> int64_t i;
> - uint16_t refcount1, refcount2;
> + uint64_t refcount1, refcount2;
> int ret;
>
> for (i = 0, *highest_cluster = 0; i < nb_clusters; i++) {
> @@ -1687,7 +1684,8 @@ static void compare_refcounts(BlockDriverState *bs,
> BdrvCheckResult *res,
> num_fixed = &res->corruptions_fixed;
> }
>
> - fprintf(stderr, "%s cluster %" PRId64 " refcount=%d
> reference=%d\n",
> + fprintf(stderr, "%s cluster %" PRId64 " refcount=%" PRIu64
> + " reference=%" PRIu64 "\n",
> num_fixed != NULL ? "Repairing" :
> refcount1 < refcount2 ? "ERROR" :
> "Leaked",
> @@ -1695,7 +1693,7 @@ static void compare_refcounts(BlockDriverState *bs,
> BdrvCheckResult *res,
>
> if (num_fixed) {
> ret = update_refcount(bs, i << s->cluster_bits, 1,
> - abs(refcount2 - refcount1),
> + imaxabs(refcount2 - refcount1),
> refcount1 > refcount2,
Hope I got that right. Here's my analysis:
Before: refcount{1,2} were both uint16_t. Promoted to int for the
subtraction. Therefore a negative result could occur. abs() takes the
absolute value and the sign is passed separately.
After: refcount{1,2} are both uint64_t. No integer promotion happens, we
perform an unsigned subtraction. The separate passed sign is okay. For
the absolute value, there are two cases:
1. refcount2 >= refcount1: No overflow occurs, everything fine.
2. refcount2 < refcount1: (refcount2 - refcount1) wraps around, but is
still an uint64_t. imaxabs() takes an intmax_t, which is signed. The
conversion is implementation defined, but let's assume the obvious
one. imaxabs() has two cases again:
diff := refcount2 - refcount1 + UINT64_MAX
a. diff > INTMAX_MAX:
We get diff converted back to signed, which undoes the wraparound.
The absolute value of the signed difference is:
-(refcount2 - refcount1) = refcount1 - refcount2
This is what we wanted. Good.
b. diff <= INTMAX_MAX:
diff is again converted back to signed, however its value is
unchanged because diff can be represented by intmax_t. This is a
positive value, so taking the absolute value changes nothing.
This is _not_ refcount1 - refcount2!
I suggest using a function that calculates the absolute value of the
difference of two unsigned values the naive way with an if statement.
Gets us rid of the implementation defined conversion, too.
> QCOW2_DISCARD_ALWAYS);
> if (ret >= 0) {
Kevin
- Re: [Qemu-devel] [PATCH v5 06/26] qcow2: Use 64 bits for refcount values,
Kevin Wolf <=